Hello sarjallen. QuickBooks does not allow you to delete or remove a vehicle once it has been added to your account.
If you no longer use the car for business, you can mark it as inactive. This hides it from your list of active vehicles and will stop the system from prompting you for mileage updates.
To deactivate a vehicle, go to the Mileage menu and select Manage Vehicles from the Add Trip dropdown. Once you have found the specific car you want to edit, you can change its status to inactive by selecting the Vehicle no longer in use option. After that, click Save.

Please note that QuickBooks requires at least one primary vehicle to stay on file. If this is the only vehicle listed on your account, the system will not allow you to deactivate it. The best way to stop the tax is to ensure the Business Usage is set to 0% so QuickBooks knows there are no deductions to calculate.
You can also check out this article to know more about how mileage tracking works in QuickBooks: Manage mileage tracking in QuickBooks Online.
If you have additional questions or concerns, you can leave a comment below.