If you set up folders for sharing, you can see who is accessing those folders and and kick any of them off at any time. Click on the Start button and point to 'Settings' and click on 'Control Panel'. Double-click on the 'Server' icon. Click on 'In Use' and a list appears showing who is accessing your computer and what files they are using. To block them from these files, click on the name in the 'Opened By' list and click on 'Close Resource'.
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.