Quite a few people on facebook are annoying. I've found them to be fair game for entertainment.
Tip: Next time they send you free bejeweled coins, return the favor. "I was going to keep this a secret, but to thank all of you who send me free bejeweled coins, here's a link that will give you 10,000 free bejeweled coins and game customization." Give them a link to that Rick Roll flash thing that you can't shut off without ctrl-alt-del, else going through the entire thing until the end.
After a day or so, "Yeah, that was annoying too, huh? Not quite as annoying as having to scroll through pages of crap to find anything interesting because every other post is some bejeweled crap."
Amazingly, your Mafia Wars friends skim right past that. Else, they're too busy posting things for that game (wall posts, so that the only way to block them is to block the person) so that they never catch on to what you just did to the bejeweled people. Repeat for them. Then, repeat for Farmville, FarmTown, MyAquarium,...
So simple - 10 seconds of work and you get one evil grin for the day. 🙂