be sure to use recaptcha for your faucet site, AFAIK bots can't answer recaptcha's captcha so they can't abuse your faucet in that way. having antibot script can help you in a way or another but captcha mostly will block those bots
Don't use image captcha, that one is very easy to crack. I could personally make a bot that does that, but faucet profits is not worth the effort.