That is the purpose of the merit system! For you see, a “platform just for Bounty” is a useless cesspool from which all of the legitimate users would flee. Incidentally, that would cause the money to stop flowing here; so don’t whine about attempts to save the goose that lays the golden eggs.
it is impossible to get spam free forum with any merit system, merit system should be used for users to be ranked in accordance with their post usability and shared knowledge on the forum, but there is no guarantee that best posts will have more merit than other posts, or that users with most merit earned are the best posters on the forum
as well, there is no guarantee that new users will not try to earn something through bounty campaigns, there will always be new users just trying to do some bounties, I do not think that you can suppress that with any rule, merit system included
but on the other hand, why do you mind spam so much? you can ignore most of those, and just go over it, without campaigns and payments they will be fast out from the forum, since payments were their primary motive to join the forum
of course, there are users putting effort to find spam and promote fight over it, and I support fight over spam, but that does not seem effective, since spam will always come with new BTC price rise, and some of new members will stay with the forum as valuable members of the community, others will go for something else
and that is how the world is going on, you cannot expect that all new members are here to love forum and post quality posts