Thanks we are gonna probably use a different saddle and the other things you said 2 do thanx for tha help!No, don't kick her. This is a issue that needs to be resolved not kicked out.
Does she have any physical problems that could be the cause if it? Does the saddle fit properly? Is your mother riding unbalanced? How does she ask her to canter?
Rule out all other possibilities before you resort to the "Just kick her" method. If it is a respect problem, go back and make sure she is listening and cooperating at all gaits before you move up to the canter. Then if all is cleared and she still does it try having your mother carry a crop and when she asks her to canter and the head goes down give her a little pop on the rump to get her attention. make sure your mother has good enough balance and a good enough seat that she can stay on when the mare does this. Don't let her have her way if she is doing it to get out of cantering, make her go with the crop and leg presure, urge her foreward with your seat as well.
If you or your mother feel like you don't have the experience to deal with it properly, I'd get a professional out to see what they think.