Pots said 'chat' so I'm assuming he means the chat room, it doesn't matter tho cos there is no way to block posts in the forums or text in the chat room.
However, it does depend on WHAT they post/type, if it's racist or abusive then best to contact a moderator and let them deal with it.
Some chat rooms i have seen have an 'ignore' feature on them whereby when your in there if you want to ignore that person you can do, it also stops them being able to whisper to you privately too.
I think it's quite a technical thing to do tho it only would only apply for the time you were in the room and would drop off if you exited.
If the block applies to the chatroom, forum etc... then it would look like folks were talking to themselves. Then I get complaints said "why am I missing text in the chatroom? John Smith seems to be talking to himself"
trust me, I know I'll get them emails!