You could get yourself a RF analyzer or an old Hammy (Ham radio enthusiast would likely have something you could borrow)and find out what they are using then that would allow you to figure out your options such as removing the antenna, sim card, or the module in some manner. The problem with removing modules could be they are tied into the cars electronic controller which could cause issues with the car even working.
Not sure about an icon or flag to depict the idea of announcing Hey I’m For Privacy!! Feels like it goes against the whole principle. It’s along the same lines of stickers on a vehicle, stick family tells (most times) exactly how many people and animals w/types are within the household, gun logos indicate a high probability of an abundance of weaponry, a privacy flag may promote someone to look deeper into you with the premise well they want privacy so they must have something to hide, mentality. I’m private I don’t need to draw attention to me being private. But you do you. Just my opinion.