Carpenters cap or many other names I've herd used.
Hope I can explain this without a picture.
You need to cut an outside miter just at the end of where you want to stop the crown.Now cut the return piece of crown to length to just cover the miter and butt up against the wall. This will cap the end with a finished look.
Be careful,this is a small piece of trim and may break or shatter when cutting.I usually just glue this piece on.Luck