I get these at UBBDev all the time, and one thing I've noticed is that it's commonly users that you can identify self registered to spam (as opposed to being a bot). Keep in mind that SFS works by users reporting spammers then they come up as flagged after so many reports; so it's entirely possible that someone wasn't listed as a spammer when they registered and are now listed after they've done their drive by's.
The latest build of the UBB should check both ip and email.
I've had bulk sets of users register and just not post as well, and they'll come back at a later date to spam.
I have a mod at UBBDev that detects users via the login screen, so every time they login it does a check.
