You could pour the excess out as long as you do a good job cleaning and sanitizing the opening. You don't want some of the trub to make it to the lip and then slide back with some unwelcome yeast/bacteria.
Another option would be to scoop it out. If you have a somewhat soft plastic or silicone cup or ladle you could scoop out what you don't need.
I don't really love any options with keg fermenting to reuse or harvest yeast. For that reason, I tend to clean out the keg and start over with a fresh pitch every time.