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.
Please note that in case of using the non-installable Agent the remote machine and Agent itself are only available for connection as long as Agent is running. Therefore, you can simply close the Agent window - this will disconnect any currently active sessions and no Viewer will be able to connect to this machine until you run Agent again. For more information please see this page on connecting via Agent.