Why will I want to prevent from ppl to PM me?
Don't ask me. I get blocking a single user, if someone should be stalking you for some reason, but blocking everyone except specified otherwise? I wonder if anyone actually does this? Of course, it is also possible to use this list to remind yourself which posters you like, for those who don't think memory alone will do the job.
There is one more possibility I just found out about. You can check which of your buddies are currently online. For this go "Quick links" and "Open buddy list".