So I've been using pidgin forever now ever since I stopped using miranda-im. A couple months back, I enabled my MSN account which I haven't logged into for a while. Since then, I've been receiving MSN IM spam everyday. EVERYDAY! Sometimes it gets so bad, it's almost on the hour. Today I finally beat the spim. I could have beaten it on day one, but I was too lazy to check up on pidgin options.
First, go to "Tools" -> "Privacy". From here, it's just a matter of selecting your account and the appropriate privacy settings. For MSN, I just changed mine to "Allow only the users on my buddy list" and Presto! I haven't received any MSN spim since!