You can stop payment on the check. However, it depends with the sale agreement that you had with the seller. If the agreement states that the car was in good condition before purchasing, then it's not possible to stop the payments.
Sure you can if they don't like it tell them to take the car back. If they protest there is always small claims court (which I have done & won a $ judgement) or other courts depending on the amount you paid. Growing up w a lawyer as a parent & studying law in college has taught me to fight when I am wronged. As long as you don't try to keep the car they can't charge you with theft & remember there is no debtors prison in the U.S. Hope you come out on top. Z