No, the spam bots don't understand captchas. But they trick a human somwehere else in the internet to answer to it. They create websites that asks for captchas to access downloads or similar, and thousands of people will be there doing the job for the bots.
Okay, I believe I found a way to prevent most of the spambots around (a few custom questions at registration). If I see the bots are not able to go past them, I will again remove the admin activation step.
Of course nothing can stop human spammers, but I don't think we have to worry about that until this place has a lot more traffic, and by then, I'll find better countermeasures.