Is it obligatory to warn the owners of cars that have Quranic verses or the Shahada (Declaration of Faith) placed on them, when these are exposed to disrespect, or is it sufficient to merely disapprove of it in one's heart?

Sticker remembrances (adhkar) should not be affixed to cars if there is a risk of them being desecrated by accumulating dust and dirt or by dogs urinating on them. This applies if they are affixed to the exterior. However, if they are affixed to the interior, they are generally safe from such desecration. There is no need to warn every individual who affixes a sticker, as desecration may not necessarily occur. It suffices to place a warning in the mosque or on social media.