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.
This is a best solution and its already happening on BTC paid campaign but with regards to altcoin bounties, there's still no guarantee even if its have an escrow especially if the escrow fund is also the token itself. Since most of the bounties are new project, most of them have no funds yet to escrow the campaign but they still able to work with some managers because they come into an agreement for sure. If in doubt, don't participate scam projects are the risk for bounty hunters you should also anticipate this one.