Event filtering and alert categorization in SIGNL4 is based on a keyword search algorithm. You can create a keyword blacklist and apply it as a filter the following way:
1. Create an alert category named 'blacklist' for your team: https://account.signl4.com/manage/Category
2. In the keyword section of this category, add all your blacklist keywords and combine it with OR, i.e. choose 'Any'
3. Now, either have all users manually opt-out from this category or if you are on a paid plan with the 'assignment' feature, opt out your users via the Assignment tab, so that the 'no signls' symbol is shown for all users.
4. Check if no user is subscribed to your 'blacklist' category, i.e. no user will receive signls for any incoming event containing any of your blacklist keywords.