I'm deleting them as fast as I can.
Priestess, it would help if every registered new account had to be approved by an administrator or a moderator before activated. Several forums, including UBB ones, introduced such a protective measure. That helps to weed out the suspicious looking accounts and undesirable IP addresses.
Another easy solution could be putting the new users on a short test period. The first five, ten (or any other number of) posts would have to be approved by the mods before they could appear on the website. That would discourage not only spammers but also trolls.
Just a thought.