1. Escrow the bounty funds and make sure it's managed by trusted managers.
2. Never join any non-escrowed bounties if you fear the bounty won't paid you in the future.
Those 2 suggestions I provide above 100% will prevent from bounty scam and make bounty scam will go away since no one is interested.
By escrowing bounty funds some times this problem is not 100% solved. Because some times the team is sending fake tokens to the bounty manager and later changing the contract address and bringing those tokens to the market. Such as Burst Money and Bintex bounty.