Yes, exactly. We added a novel image rotation puzzle to registration a while back which helped for a bit, but was eventually cracked. I see the math puzzles around many places now and if they are not yet cracked, they will be. One problem is that in fact many of these spam registrations are *not* bots but instead extremely cheap labor, organized over the Internet, often from 3rd world countries or other places where cheap labor is plentiful (unfortunately services like "Mechanical Turk" feed this industry). Since they are real humans, they can usually solve these things. The only barrier that stands in their way is making it take so much time to solve the puzzles that it's not worth it for the pay they get. This of course also means it takes your real visitors more time, but as long as it's not so high that they leave, then it's a win I guess. It's a tough balancing act.
- Oshyan