I'm by nature a bit of a pacifist and am specifically going out of my way to take the high road and have nothing I do have the chance to come across as spiteful, so you can take this with as big of a grain of salt as you want. Do you have to pay your insurance month to month or 6 month? If month to month: I would just send her something like "Hey, I have to update my insurance policy and am going to remove your vehicle. I know it is short notice so if need be, I can pay for March but afterwards, it will be your responsibility. If 6 month: same phrasing except: I'm sorry for the short notice but it needs to be done. She can try to spin that as spite but I think it would be a tough spin to make.
M:26 H:32 T:8yrs ILYBNILWY/ "I'm not happy": 2/6/16 D-Day-discovered PA/EA: 2/10/16 Separated: 2/14/16 He's living with OW I moved to different state: 06/16 Currently: Trying to move forward