You should better focus on improving your ad revenue to compensate loss from bots, if any. Fighting against bots is a waste of time IMO.
This is what I have observed! Only high traffic compensates losses caused by bots.
But I would be grateful if someone can say something about changing captcha - it can help or not? My guess is that it can be difficult for bot owners to change options everytime when you change claim time and captcha system.. or..?