So, the admin can add the fifth group, which member in this group have no access to bounty marketplace
ex:
5. Limited members.
And how to moving spammer to the group? I think global moderator and some of another moderator have right to add/remove them in this groups, based on moderators reports and/or just find it.
That still doesn't solve the issue of them spamming the board. You can claim a bounty anywhere online. People will just use social media instead of the bounties subforum. If they aren't contributing useful content, get rid of them.