You can disconnect the Viewer in a couple of different ways:
1. By stoping the Host service. You can do that by using the "Stop Host" item in the context menu or in Windows Start -> Remote Utilities - Host -> Stop Host. You can also stop the Host service (rutserv.exe) in the list of processes.
2. By clicking on the X button in the notification panel (provided that option to show notification panel is enabled in the Host settings).
Stopping the Host like described in p. 1 is available only when you are an "administrator" on the remote machine. That is, a "user account" won't be able to stop the process without admin rights. However, p. 2 is always available for any account.