Need to create a license file for homegrown app

We have a 10 year old homegrown Access application that we provide to customers at remote locations, mostly nursing homes. When we lose a customer (they decide to use a different distributor), we need a way to remotely disable the software. I would like a way to create a license file that they need to update at certain intervals, or other suggestions on how to accomplish this goal. The application was created with Access 97. We actually own some of the computers. So with those specifically, I have considered using something like “Logmein” to remotely access and simply alter the executable. But on the ones that we don’t own, does anyone have any suggestions?