As i stated before you should just check who was doing more than 18-20rolls(you decide) per day more than 5 days per month last few months(if you can do that). 99,9% of those users are bots or exploiters like mewbrrj(2 users working together) and 0,01% insomniacs. If you cant at the moment maybe you should try to.
You must have missed my reply in the previous post. They would just amend their bots to claim whatever number we set it to.
The bottom line is that the only way we can stop botnets is to make the reward less than they pay for a captcha solve.This is true, but limiting the max amount of claims per day would not trump normal users (except the guy who tag teams with his mom, haha) and it would surely trump botnet users. You could limit it to 22 claims per day for example (which is a lot of money if you multiply those 2 claims by all the bots).
That extra money could be kept as profit or it could be given to users in the form of a bonus.