Key Takeaways:

• Blocklisting (formerly called blacklisting) works both ways: advertisers block websites and publishers block advertisers

• It's primarily used for brand safety, fraud prevention, and audience protection

• Both sides need to maintain their blocklists regularly to stay effective

• Most platforms now offer category-level blocking alongside domain-specific controls

What is Blocklisting in Digital Advertising?

Blocklisting is a fundamental safety mechanism in digital advertising that gives both publishers and advertisers control over what appears where. Think of it as a digital bouncer for your ad inventory or campaigns.

For advertisers, a blocklist is a "do not serve" list of websites or apps where they don't want their ads appearing.

For publishers, it's a "do not accept" list of advertisers, creative types, or entire business categories they refuse to display on their properties.

This two-way street is crucial to maintaining quality and trust in the programmatic ecosystem.

The Advertiser Perspective: Where NOT to Show My Ads

Advertisers use blocklists for three primary reasons:

1. Brand Safety

Nobody wants their family-friendly product advertised next to controversial content. A notorious example happened in 2017 when major brands found their ads displayed alongside extremist videos on YouTube, triggering a massive advertiser exodus.

Brand safety blocklists typically include:

• Adult content sites

• Violent or graphic content

• Politically divisive websites

• Misinformation or fake news sources

• Low-quality content farms

2. Fraud Prevention

Ad fraud costs the industry billions annually. Blocklisting helps filter out known sources of:

• Bot traffic (non-human impressions)

• Click farms

• Sites with suspicious engagement metrics

• Domain spoofing attempts

As Jack Smith at MediaMath explains, "Recognizing and blocking fraudulent traffic isn't just about saving money—it's about maintaining the integrity of your campaign data."

3. Performance Optimization

Not all inventory converts equally. Blocklisting helps exclude:

• Sites with historically poor conversion rates

• Apps with high rates of accidental clicks

• Websites with bad viewability scores

• Placements misaligned with target audiences

The Publisher Perspective: What Ads NOT to Accept

Publishers have their own reasons for implementing blocklists:

1. User Experience Protection

Publishers work hard to build trusted relationships with their audiences. Certain ad types or advertisers might damage that trust:

• Disruptive or aggressive ad formats

• Misleading claims or deceptive offers

• Low-quality creative execution

• Excessive animation or auto-play audio

The Washington Post, for example, blocks certain interactive ad formats that overlay content without user initiation.

2. Competitive Conflicts

Many publishers block direct competitors from advertising on their properties:

• News publishers block other news outlets

• E-commerce sites block competing marketplaces

• Subscription services block similar offerings

The New York Times famously maintains strict competitive blocklists for their subscription business, preventing other news subscription services from targeting their readers.

3. Content Policy Enforcement

Publishers often have specific policies about what products can be advertised:

• Family-friendly sites block gambling, alcohol, or adult dating

• Health publications might block unverified supplements

• Financial sites block high-risk investment schemes

According to Google Ad Manager's sensitive categories documentation, common publisher blocklist categories include gambling, alcohol, politics, and weight loss products.

How Blocklisting Works in Practice

For Advertisers:

1. Identify problematic sources - Review performance data, use brand safety tools, or subscribe to industry blocklists

2. Create your blocklist - Compile domains, apps, or IP addresses in the required format

3. Upload to your DSP/ad platform - Most DSPs have dedicated blocklist sections

4. Monitor and update regularly - New problematic sites emerge constantly

For Publishers:

1. Determine unacceptable advertiser categories - Align with your audience expectations and content policies

2. Configure SSP/ad server settings - In Google Ad Manager, this is done through "Protections"

3. Block specific advertisers or creatives - Either by domain or by creative ID

4. Review regularly - New advertisers appear daily

Blocklisting vs. Allowlisting: What's the Difference?

While blocklisting tells platforms what NOT to accept/show, allowlisting (formerly whitelisting) takes the opposite approach:

Most experts now agree that allowlisting provides stronger protection, but it's also more restrictive and requires more work to maintain.

Many publishers and advertisers use a hybrid approach:

• Allowlisting for their most premium inventory/campaigns

• Blocklisting for broader reach with basic safety measures

Common Challenges with Blocklisting

Blocklisting isn't perfect. Here are some challanges you'll face:

1. Constant maintenance needed - New domains and advertisers emerge daily

2. Coverage limitations - Impossible to identify every potentially harmful source

3. Over-blocking - Too aggressive blocklists can severely limit revenue or reach

4. Implementation complexity - Different platforms use different formats and rules

A study by Confiant found that even top publishers with sophisticated blocklists experienced malvertising incidents, showing the limitations of this approach.

Best Practices for Effective Blocklisting

For Advertisers:

1. Use dynamic, regularly updated lists - Subscribe to industry blocklists

2. Combine with contextual targeting - Add another layer of protection

3. Review performance impacts - Ensure blocklists aren't overly restricting reach

4. Consider category-level blocking - Block entire site categories rather than individual domains

For Publishers:

1. Balance revenue with user experience - Too strict and you lose money, too loose and you lose trust

2. Implement category blocking - In Google Ad Manager, use both general and sensitive category blocks

3. Maintain advertiser-specific blocks - Keep lists of specific advertisers to block

4. Monitor user feedback - Users often report bad ads before your systems catch them

Real-World Examples

Publisher Blocklisting

• News Publications: Many news sites block political advertising during election seasons to maintain neutrality

• Children's Content: Gaming sites aimed at kids block alcohol, dating, and gambling verticals

• Premium Publishers: The New York Times and Washington Post maintain extensive blocklists of low-quality advertisers and aggressive ad formats

Advertiser Blocklisting

• Luxury Brands: Companies like Rolex and Louis Vuitton maintain extensive blocklists to ensure their ads only appear in premium contexts

• Financial Services: Banks typically block cryptocurrency, gambling, and adult content sites

• Family Brands: Consumer packaged goods companies maintain strict blocklists to avoid controversial content

The Future of Blocklisting

As we head into 2025, blocklisting is evolving:

• AI-powered screening - Machine learning now identifies problematic content before serving ads

• Real-time blocklist updates - Dynamic systems that update automatically when issues are detected

• Contextual renaissance - With the decline of cookies, contextual blocking becomes more important

• Standardized approaches - Industry initiatives working toward common blocklisting frameworks

Final Thoughts

Blocklisting remains an essential tool for both sides of the programmatic ecosystem despite its limitations. When implemented thoughtfully alongside other safety measures, it helps ensure ads appear in appropriate contexts and publishers maintain quality standards for their audiences.

For beginners in ad tech, understanding blocklisting basics is an important step toward creating safer, more effective advertising environments—whether you're buying or selling.