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.
If from the start the project really wanted to scam, the escrow entrusted to the bounty manager does not really guarantee that the token will be used for the project they are running, for example they provide a token that is different from the smart contract of the project they are running.
the point is we have to really pay attention to the bounties we are working on if something looks weird ask the team, if they don't satisfy you, leave immediately before the work feels useless.