Maybe this isn't a good idea for everyone, but here, almost all the spam signups are something like this: tanya906828. Normal real people don't use a name followed by 4,5 or 6 numbers. Most don't bother with numbers at all. Could there be a plugin that will allow me to foil signups that contain more than a selected number of numbers in the name?
I get 30-40 a day of these rascals, and would be ecstatic if they could be prevented. It would be fun to inform them that their name has already been taken so they don't catch on so fast to the fact that it was blocked from signup.