What do you mean by "delete a priority program?"
Do you mean, "how do you remove a program from the priority list"?
If that's what you mean, then I think you have to remove the series recording itself.
As far as I know, there are only two ways to do that - if someone knows another way, please let us know, as I for one would like to know this as well:
(1) If the show is on the guide, highlight an episode scheduled to be recorded; one of the menu options should let you remove all future recordings.
(2) If you have any episodes still on your DVR, go to the "Recordings" list, then find the series, then highlight the "X" on the right side of the screen; this gives you the option to remove future recordings, although it also deletes any existing episodes that you recorded.