Your question shows that you are interested in bounty hunters specifically. What I can say is that you can offer participation to members with a certain amount of merit, thereby limiting the number of bots that will spam the forum. In fact, there are a lot of cheaters who are engaged in bounties, and if you need to advertise the project on social networks, then rejecting newcomers who do not have merit will be the best choice.
As far as subscription companies are concerned, most of the users will participate in payment in BTC, and it seems to me that if you want good posters, then the payment for posts should be decent.