If you have a truly willful child, you will have seen stubbornness and other signs before now. They don't usually begin to appear just at this time. If you have seen the signs before, look back and remember how you've handled your stubborn child who sometimes digs in his or her heels and refuses to cooperate.
Sometimes a 3- or 4-year-old child stubbornly refuses to make a bowel movement in the toilet or potty and insists on diapers even when the child uses the potty or toilet appropriately when urinating. Sometimes a child even insists on taking underwear off and putting a diaper on 4 just before a BM. It's frustrating because it's obvious the child feels the sensation of the coming BM and can exercise control to go when he or she wants to. In addition, children who retain BMs for any great length of time are more likely to become constipated. Sitting a child on the toilet for long periods is not necessary when a child understands how to use the toilet. Children who have been lectured or reminded too often, and forced to sit on the toilet for long periods of time against their will, may understandably be resistant to your pressure when control is the issue.
There is no single solution to this situation. For a child who might be stuck in this behavior and for whom even rewards don't work, and you don't think constipation is the problem, change your approach. Back off for a few weeks. Wait for a stress-free week, and then tell your child that when this box of diapers is gone you're not having any more diapers.