A famous gang (and good beta testers devoted to find bugs abuse) highlighted a very interesting game failure:
In Town Control, they spawned with 65 mercenaries.
A GM was observing the scene and took measures: he put in jail 13 mercs leaders.
BUT in few minutes, they have the ability to register 20 new characters and have again 50 mercenaries over town and ready to spawn.
SO, my suggestion is to add a condition to hire mercenaries: lvl 21 requested.
In that way, even if you use proxies and create new mercs leaders characters, you will have to lvl up before striking again => I think it will be sufficient to avoid such abuses.
And please don't remove mercs because of this game abuse!! => Having mercs is a very interesting feature in this game, would be bad to break it.