Ehm, changing the ownership of the towers? Move the shops?
Those are the obvious ones. I don't really know how to do this, you might want to try de-select on your towers every 0.2 seconds or so... It will work somewhat good but wont be bullet proof.
I have a similar problem that I feel would have the same solution.. I don't want flying units to be able to purchase from a tavern on my map... so if anyone can help, please do!
Um, I don't got the editor opened and I don't know if this will work.
IF the mercenary shops have got the shop sharing/select hero/unit ability, you can modify the ability, changing Interaction value to one or more of the following: