I've been getting about 4 or so members a week signing up with usernames starting in a single lowercase letter then two names starting with an uppercase letter. Like "vVaughnRosalesm", "jLeonardoYatesk", "sLanceBuchanana" or "kRickiCharlest". Every one of these users that I have looked up on Google comes up with a bunch of random forums they are members of and "stop spam" websites.
The weird part is they don't post, don't have any details listed, and I can't see how they are spamming anything but for the fact they are obviously not real people (or at least people interested in the subject my forum is on).
I have Anti-Spam Verification Questions installed and have three questions that you would need to know about Subaru's to answer. That filtered out most of my spam and all of the robot spam. But these must be real people.
Not sure what to do about them...
there is no way to stop human spammers, you can try but most likely they will find a way at some point to register on your forum. you can try something like httpbl and the honeypot project it might help a bit.
Ah, i've been hit a few times by human spammers... the worst kind! It's impossible to stop them, as they are not robots that you can automatically detect or predict the way that they'll react. Thankfully, i've only seen a few of them on my forums... Personally, i have been using only one anti-spam mod and it has been working extremely well. Zero spam bots yet, and it's been 5 months since i installed it.
http://custom.simplemachines.org/mods/index.php?mod=2182
I hope it works for you, but like i said, human spammers can always circumvent this protection or any, for that matter.