So say there is a user who's quoting you all the time, basically harassing you. If you could block the user, then they couldn't PM you, (You couldn't PM them,) they couldn't quote you (You couldn't Quote them,) And they couldn't reply in your topics. (As you couldn't do it in theirs.) I know that it probably won't happen, since this isn't an official SMF Message to the SMF company, but I'm just saying.