I'm not sure if someone already mentions this.

As we know that spam posts are from:
1. Noobs who pursue rank: after they reach a certain rank they can do some bounty
2. Sign camp participants who perform tasks without care what their posting, just to pass their post target.
3. Some of them have several accounts to do the bounty.

To deal with it actually, we can use the basic functions of the SMF feature.

I believe that the admin only applies 4 types of member groups in this forum, they are:
1. Global mod members
2. Local mod/sub mod members
3. User members
4. Guest members

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 they just find it.  

Reference:
https://wiki.simplemachines.org/smf/SMF1.1:Membergroups
https://wiki.simplemachines.org/smf/Moderation_center

edit:
About merit, I agree with this:
What you think about give merit to the post that does not exist?