Simply charge a fee to submit a report. At 1% of the payment for low bounties it's perfectly valid. Maybe progressively scale that down a bit as the bounty goes up. But still for a $50k bounty you know is correct it's only $500.
But I do think established individual and institutes should have free access ; leave a choice between going through an identification process and paying the fee. If it's such a big problem that you REALLY need to do something ; otherwise just keep marking as spam.
Also I've heard many times cases when company refused to pay bounty for any reason.
And taxes, how you'll tax it internationally? Sales tax? VAT?
i suspect 1usd would do the job perfectly fine without cutting out normal non-american people.
Pick someone already rich so the reputational damage from stealing your bounty exceeds the temptation. The repeat speakers list at defcon would be a decent place to start.