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.
Would it be outrageous to red-tag the bounty managers with negative feedback for showing untrustworthy behavior by accepting obvious alts and shitposters for their campaigns? I personally find this type of behavior to be untrustworthy... They are just allowing users into their campaign to maximize visibility for their products/clients, and they blatantly turn a blind eye to shitposters and copypasta to make and pay a quick buck. Not okay, in my book.
Negative feedback doesn't deter all members from dealing with that specific account, but at least it will let bounty managers know that they need to take their responsibilities more serious.
Just a thought.