If you set up a Google Tag Manager blocking Trigger / Exception Trigger, and it doesn’t block your tag from firing, then the reason could be Tag Sequencing.

Imagine a scenario where:

• Tag A – is a tag that’s configured to run with a mandatory Set-Up Tag B
• Tag B – is the tag that you want to block with an Exception Trigger, and
• Trigger C – is your Exception Trigger

In this scenario, you will see that even though Trigger C fires, and Tag B’s exception trigger parameters are all met, Tag B will still fire. That happens because it is set up as a “Set-up Tag” under Tag A’s settings like this:

So, if your Google Tag Manager blocking trigger is not effective for this reason, your solution is to remove the Tag you want to block (Tag B) from being a “Set-up Tag” for any other Tag. You can still edit Tag B to specify that Tag A needs to fire after it fires – this will maintain the sequence without messing up your blocking trigger.

Here are a couple of other great resources about troubleshooting Trigger Exceptions. Neither of them describes this exotic scenario:

https://www.analyticsmania.com/post/tag-in-google-tag-manager-is-not-firing/#blocking-trigger

https://www.bounteous.com/insights/2020/06/23/trigger-exceptions-google-tag-manager/