If you are making jokes or offence/antagonism about a member of the site, it'll be removed as member post (regardless of whether you mentioned him/her). In that case, I don't need to @mention a user, his/her name is enough.
I'll add: It could also be if you were trying to incite people to talk badly about other users, even if you yourself don't mention a specific user, such as asking, "Which GAG member is the most annoying?"
It's when you do something like " @Nathand is a goofball " just with harsher terms. Just don't give out specific names when you're rambling about how much of a ___ someone is for disagreeing with you or so.
It is a category to prevent you from ragging on another user or someone by their real name. You see them on here daily. One yesterday was, "Who thinks that XYZUSERNAME is an idiot and knows nothing about shit" I can understand GaG not wanting that kind of crap.