Usually every new field in the registration keeps bots away. I haven't checked what you did, but I saw a few forums introducing "check this is you are human" boxes, and since bots don't know the boxes, they fail at the test.
Anything nonstandard will do.
Problem is, modded forums are harder to upgrade thus I did not try that with my forum yet - I recently changed to "approve by admin", so I went just the other way than you did
