I wouldn't remove reCAPTCHA before you have a really good solution (I don't know any). If your solution isn't good enough people will write bots again, CoinAd will loose money, you have to cut the prize size(s) again and a lot of the users will not use it anymore. My advise is: Don't remove reCAPTCHA even if these images are hard to solve sometimes.
If I don't land on a larger prize, I don't even bother to fill in the captcha for the small default prize because it's not even worth my time. And I'm guessing that's when most people will be receptive to clicking on ads, after they've finished refreshing the page many times and submitted the form. So a captcha that's too much effort for a human will also decrease the ad revenue.
Coinvisitor.com just went through a similar captcha adjustment. They had a sane captcha, then some joker said he could bot it, coinvisitor over-reacted by making it impossible to solve by humans and bots, then eventually reverted back to something saner after enough people complained.