As our board (SMF 1.1.14) has lots of regular spam users, I'm trying to find measures that have an impact on them. However, so far no success in this regard.
One of the things I find strange at the Registration is that the visual verification image actually always displays the same picture, even if you click the button in order to generate a new one. Does anyone else have this experience as well or knows a reason for this behavior?
In short, yes it does stay the same. ;)
Best bet is to look in the modifications section for anti-spam mods. One I've used with great success is Verification Questions (built-in to v2.0)...
http://custom.simplemachines.org/mods/index.php?mod=1516 (http://custom.simplemachines.org/mods/index.php?mod=1516)
Not much you can do against a determined human spammer though apart from enable Admin Approval / Member Approval.
Thanks for the reply, YogiBear! However, we already have Anti-Spam Verification Questions installed (posing one question at the moment), along with Stop Spammer and Login Detector. The registration also requires that you need to confirm your registration via e-mail, but nevertheless it all has practically no effect whatsoever. I also don't think the spammers are actually human, because they register with precise regularity...
try using more then one anti-spam verification question, it should help greatly.