Many years ago United used to have a feature online that let you "cancel check-in" yourself. It was very convenient if you needed to make a change to your reservation. I'm not sure if they still have that feature.

Echoing what others have said, it is possible, but it must be a PITA for the agents because I have been told numerous times that it wasn't possible to cancel a check-in. I've only been successful by hanging up and getting another agent, or by being firm and pushing back.