A solution has to go further then rank.
Do you think as a bounty manager or campaign manager, KYC will be better?
If yes then managers has to implement a KYC system to limit those farming accounts in a single campaign. I don't if a user can actually provide about 9 to 15 separates KYC details to receive payment. The process can be done manually and the kyc details has to be recorded in a separate sheet, this will take 10 to 30 second video video to prove real ownership otherwise no way to prevent scammers or cheaters.
Most at times if KYC is implemented you will notice they always apply with their real account before anything else.
You could implement KYC into a bounty but then your participants shrink to nearly 0 IMO. I don't think many bounty hunters would be interested in sharing their info. Definitely would help in solving the problem though.