There is some Russian syndicate that is spamming all forums, they use some software to find forums, then they actually manually register an account to start spamming. I know because I manage a forum for a client and we've received an increased amount of fake registration attempts recently, and they are usually from .ru accounts.
Patching won't help. The trick is to make it more difficult for them to register and to start new threads.
You can start by having an email verification system for registration, so they need to receive a verification email and click the link before they can login.
Then for posting, just set a minimum number of posts before they can start a new thread, maybe they need 10 posts before they can start a new thread.
The first should be available as a free plugin for SMF, the 2nd should already be a feature.
This won't stop everything, of course, but it makes it more troublesome for them to spam.