Take a look at the interesting idea MotionCAPTCHA.
I'm not sure that it's ready to use (at least it should work on the server side), but I think it is possible to use this technology to combat bots.
I think to successfully implement is necessary:
- Generator pictures (order to prevent reuse of a response)
- Analyzer entropy (it may be possible to find a filter's formula to separate a human entropy from a computer)
- Maybe something else, it is necessary expert opinion and brainstorming