The only think that I know so far even though it is not popular is KYC, it is very hard to have more than one ID from your government even though somebody might simply steal it from other person (but this is very risky to do).
One of the reasons KYC is not so unpopular is because of identity theft. On top of this, am pretty sure there are lots of scam airdrops that had a requisition for KYC that unsuspecting bounty hunters joined and ended up having their ID data collected. These same ID's are used to campaigns using multiple accounts
So all in all, i don't think you can completely weed them out but you reduce the cheating whilst trying to maintain the number of genuine bounty hunters who would be interested in your bounty.