Group Policy - sharing tab

Are the users administrators on their local systems? If so, they can override any GPO setting, although it might make them more difficult.

A cheap and easy fix is to disable the server service on the workstation, which will prevent user's from creating shares. It will prevent any shares on the workstation and could have an impact on any applications, such as RDP, that require shares.

Here is how to disable it.

Click Start
Point to Settings
Click Control Panel
(Windows 2000 only: Double-click Administrative Tools)
Double-click Services
Double-click Server
For Startup Type, select Disabled
Click Apply
Click OK

then you could use Group Policies to disable access to services