I would consider blocking all countries that aren't the U.S. and Canada (with a couple of exceptions if known members are from a certain country).
I don't think we have anyone on here from China or Thailand, but.. that's where a lot of spam comes from, so......

Also, consider limiting PM rights to actual members by restricting that ability to members with 3 to 5 posts as a minimum before that permission kicks in.

http://www.vbulletin.org/forum/showthread.php?t=168305