When both solutions are bad, one has to choose the better solution even if its far from being perfect. Yes, making a merit/rank requirement for bounty participation will create a whole new black market of merit trading (if it doesnt already exist...). It will probably have other unwanted effects. However, it will reduce the spam. Dramatically. I believe spam is currently by far the largest and most annoying problem in BTT. Solving it should be top priority even if new issues would arise.
You have no idea of the amount of money these shitposters make from the bounties sections. Take a look the alt accounts reporting thread in "Reputation" and you will see the shitposters who became millionaires by abusing bounties with their alts. Of course every third world beggar will come to this forum to continue this legacy and get paid handsomely in return. Who would want to do a 9-5 Job for such?
Spam is a problem for the forum but its also the job of the bounty manager to keep track of such people. Most of them (except a few) dont do their job properly.