Add the following to your logon script:

<b>if exist %windir%\scrnsave.log goto scrsavok
regedit /s \\<ServerName>\<ShareName>\scrnsave.reg
@echo JSI > %windir%\scrnsave.log
where \\<ServerName>\<ShareName>\scrnsave.reg contains:

\[HKEY_CURRENT_USER\Control Panel\Desktop\]

The NoDispScrSavPage entry is from tip 0070, and will prevent the users from changing your settings.

NOTE: Change ScreenSaveTimeOut to be the time out that you want and SCRNSAVE.EXE to be the full path to the screen saver you want to use.