Every dealer has a few “Problem Accounts” that cause false alarms, excessive service calls, and serious customer dissatisfaction. It’s probably not your fault, but they are still “Your Problem”. Bite the bullet and fix them no matter what it takes. Maybe you can charge a little for the repairs but do it anyway even if it’s for free. In the long run, it’ll be worth it.
If you don’t know what the problem is, study the account until you figure it out. Talk to the customer, review the files and history. Do whatever you need to do to fix the problem. Otherwise, “invite’ the customer to go elsewhere. They will probably cancel, anyway, the first chance they get, and they’re certainly not giving you good referrals. You don’t need the hassle and potential damage to your reputation.
Sell the customer a service agreement. You can fix the problem and increase your RMR at the same time, truly a win-win situation.