There is no way to "Un-enter" a sweepstakes, because program would not handle "zero times entered" thing. However, you may easily tweak the program by using Entered Date field. For Daily sweeps you'd change Entered Date to yesterday, for Monthly - to last month, etc. For single entry sweeps you may change Frequency to Yearly and Entered Date to last year.
BTW, same way you may handle sweepstakes that start on future date.