Have you tried switching your Image Verification over to the
reCAPTCHA system?
My site isn't what I'd call big, but it did decrease the amount of spam bots trying to get in when I started using it.
You could always try the
NoSpam! hack, it seems to be good. I can't give you a personal opinion however as I haven't needed it.
In addition, that
"Stop the Registration Bots" hack isn't the best choice.
Some bots are timed to do the registration in human time and there is a FireFox Modification that automatically fills in all the detail fields for you with defaults that you set.
So, in reality, it might stop some bots, but it could block out some members to.