Can an end user stop an On-Demand Assist session?

Yes. On Windows and Mac, the end user retains the ability to control the keyboard and mouse while a session is in progress, and can click the always-available End Session button at any time:

On Android and iOS, the end user retains control of the device while a session is in progress as the technician's connection is view only. The end user can end the session at any time by opening the VNC Server app and tapping Disconnect.