You can just hide a person's posts. I do that with all my Mafia Wars "friends" who aren't friends with me otherwise. I could just delete them after we join each others' crews, but I would much rather be able to use them for other games as well.
I have few lists with varying degrees of privacy settings. Game people I have on the tightest privacy. They see a few things of my profile, my friends, but not my photos, feed, etc. I have one for HS and Uni friends I haven't kept in touch with/don't want to get back into touch with or wasn't that close with to begin with. I have my normal list, then I have my "closer friends and family" with contact details and so on.
If anyone gets too spammy, they get hidden unless they are my sister.