Add the following to your logon script:

if exist %windir%\scrnsave.log goto scrsavok                              regedit /s \\<ServerName>\<ShareName>\scrnsave.reg                              @echo JSI > %windir%\scrnsave.log                               :scrsavok
where \\<ServerName>\<ShareName>\scrnsave.reg contains:
REGEDIT4                              \[HKEY_CURRENT_USER\Control Panel\Desktop\]                              "ScreenSaveTimeOut"="360"                              "ScreenSaveActive"="1"                              "ScreenSaverIsSecure"="1"                              "SCRNSAVE.EXE"="C:\\WINNT\\System32\\scrnsave.scr"                              \[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\System\]                              "NoDispScrSavPage"=dword:00000001
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.