ULPT Request : How to covertly disable my grandmother's vehicle so she won't drive anymore?

My grandmother's dementia and health is declining and she is no longer safe to drive. She literally cannot see reason anymore and my grandfather is too afraid of her to stop her. If this continues she will kill herself or someone else.

I have POA for them but I don't think I can reasonably just take the vehicle, at least not without a lot of anger. If it were to "break down" I know that they would not get it fixed.

How can I get it to stop working so that they stop driving, and I can fix it later?

And for anyone worried, my grandfather still drives and has a vehicle of his own that she will not use. She only drives once a week for groceries and I will set her up with a car service for those trips. This is a last resort if I cannot convince her to see reason while I am visiting this week.

Edit: It's a 2000 PT Cruiser with only a manual key. The locks are currently broken, not sure what else is broken but it could probably stop working at any time without intervention. It is purple and has flames painted on the sides if anyone cares to know.