Some time ago we were working on validation on one czech programming forums that required solving some kind of equation to let new user join & post (e.g. something like mathemathical captcha). It's though not running anymore (we made the forums private).
Of course for the user it's not a problem to solve (or put that to wolframalpha). But that's something that a bot won't do.
Maybe a good idea to implement ... of course ideas are one thing, finding time and implementing another (where I sadly can't help till late June)