How to stop a group of users reporting messages?

[Videx]

Of course, if they're banned they shouldn't be able to report posts anyway. But I guess you mean when they get back?

In our rules we include abusing the report-a-post function, so we would just extend their ban for doing it.

Edit: I forgot you mentioned Unregistered Users can also do this. Your mistake is allowing them to post. Turn off posting for Guests and they won't even see the report-a-post button.