Since it is software based and you are concerned with them initiating a session, you can simply have the program/service be stopped on the machines that they would try to connect to. With the software turned off on your end, they will have no way to connect to those machines. When you need them to connect to those machines, you can just start the software back up.

If you have not already, I would bring your concerns to the company that wants remote access. Don't be afraid to tell them you are paranoid about security (since you are responsible).

Good luck!

~Halv