First you need to look at the business model. If the systems are that hard to troubleshoot, then maybe it is time to start upgrading them. Is the the only guy in town that can troubleshoot these? If you can not fix them, can other dealers?
Having your business hinge around 1 employee is something I would avoid. What if the guy gets hit by a truck?
I was in the office equipment business and we had this particular fax machine that was just a piece of garbage, and we had a bunch of them under contract. When the annual contracts came up for renewal, I raised the contract price, and gave a killer deal on a new model with warranty etc. I think we converted every single one to the new model.
I broke even on those deals, but it in the long run I saved money by avoiding time consuming repairs, and customer perception improved (doesn't matter the machine is garbage, we service it and when it breaks, it is our fault)
I think you want to know how to get the guy to stop, and that doesn't seem like it will happen.
I would look for ways out of this. Train your new techs, start upgrading equipment, etc.
And if the guy is that irresponsible to disregard your repeated warnings on the phone, who knows what else is going on.