If you set up folders for sharing, you can see who is accessing those folders and kick any of them off at any time. Click on the Start button, point to Settings, and click on Control Panel. Double-click on the 'Administrative' icon. Double-click 'Computer Management', then 'Shared Folders'. Click on 'Sessions', then right-click on the name of the user you want to kick off and select 'Close Session'.
NOTE: Any files that are being accessed by a user when they are disconnected could be damaged, so be sure to do this when they are not connected.