I'd expect that they will give you a hard time and send their collection agency after you. I do not know if you have any real defense, other than fraudulent inducement, or if Mississippi has a consumer protection law allowing you to back out. Contact the MS consumer protection department and ask them -- it may be in the Attorney Genera's Office.
Most companies treat people fairly and don't make a big deal if someone immediately wants to cancel BUT many of these companies are con artists and try to take people for a ride. They have a whole set of procedures when someone signs a contract and backs out.
Make sure the stop payment is in writing and is renewed after it expires as they'll possibly try again to deposit after the stop payment order lapses. You may benefit from a Legal Insurance Plan for a local lawyer who can help you plan the defense you'll need.
This is intended as general information only, NOT legal advice. You are not my client and I have no obligation of any kind to you. To retain a lawyer I suggest you go to www.AttorneyPages.com.