I had the same problem to.
I just added another question during registration in order to get rid of these, and seems like working.
Good think is that is a Greek forum and the question is in Greek language so is not so easy for spammers.
I did not use any hacks or plugins, I just added another field in users according to this....
http://www.vbulletin.com/forum/showthread.php?t=205214