

Add a submission fee that gets refunded as part of the bounty payout, or if the reviewer otherwise judges the submission as obviously legitimate.
Donate all fee proceeds to charity, if you want to counter the any incentive to deny submissions for financial gain.
Every situtation is different, but in general, It’s the same as the “innocent until proven guilty” concept: I’d rather give money to someone whose being dishonest than not give it to someone who really needs it. Cause I appreciate how close ANY of us are to homelessness and destitution in this world. My own generosity is my own generosity, not determined by the honesty, or lack thereof, of others.