I have created a new stylesheet but now I'd like to delete a style type, for example "program" from it. How can I do it ?
I probably haven't understood your problem, but since you're not going into Mr.Wizard's question I have to go by what I read. So here it goes.
Here we have a MMA notebook (left) and a style file (right):
Deleting a style from the style file (just selecting the cell bracket and hitting the delete key):
And finally deleting the last style, we're back at the default style: