With offender warnings you can choose to show a popup message on your website to website visitors clicking on your ads that trigger specific protection rules.
How to enable offender warnings?
In order for a popup warning to show you have to explicitly enable the "Display popup warning to visitor" action, within protection rule:
How to configure offender warnings?
You can customize the text your popup warning will show in Account Settings. Here's an example of the default warning text that can be shown to offenders:
Additionally, you can customize the visual theme of the popup warning:
How to disable offender warnings?
You don't have to disable them - they will be shown only if you explicitly configure them in your protection rules and only if the rule was triggered.
Can you use HTML?
You can basically inject anything into your website when the specified rule is triggered.