The following design is possible:
1) miner ONE (human) makes a captcha and the client app submits it to a pool server
2) miner TWO (human) gets a random captcha from a pool server and submits back the answer
3) random p2p clients (human) get both the original captcha and the answer and verify that the answer is correct
4) if the answer is correct a block is created and miners ONE and TWO get a reward
5) p2p clients also get a small reward but for just verifying (kinda transaction processing), regardless of whether the answer was correct and their intention is to keep the blockchain healthy as only then their earnings make sense
1) Botpool 1 generates a huge number of captchas and submits them to the pool server
1a) It also passes all of the generated images and solutions to botpool 2
2) Botpool 2 repeatedly requests captchas from the pool server. If the image matches one it already has from botpool 1, it submits the correct answer, otherwise it just burns it and requests the next.
3) Some poor sods have the job of verifying all this bot generated work for almost no reward
Computers could generate and match these images so fast that no human actors could possibly compete.
1) to submit and answer captchas you need to register and the submission rate is limited
2) coins could vest and if malicious activity is found out then all non-vested coins get confiscated