Please look up fecal impaction and encopresis before you blame the child and say it is a battle of wills. Very few 6-year-olds are without bowel control simply to be spiteful. She probably has a disfunctional colon and lower intestine, and that is a medical condition, not being willful. Google "fecal impaction" or "impacted bowel." There is also a good Parents' Magazine article from March, 2010, if you can find a copy at the library, that is called "When Your Child Just Can't Go." Look it up.
you say a battle of wills, have you tried taking a step back and letting her be in control of her bowels ,what exactly is the issue does she not use the toilet? more information may help .