That is a tough one. Have you asked over in the
group policy newsgroups ? However, I will admit
to tending those newsgroups now and then and I do
not know of a policy to effect what you seek.
Since what you want to do is prevent the users from
deleting the Scrnsave.exe reg value in their own
HKCU/Control Panel/Desktop reg key, and to require
that there must be one, but there are only permissions
on keys and not values, it does not seem you can do
this by manipulating reg settings/permissions (unless
someone knows of an overriding reg setting).
Given all of the above is so, then the only way I can
think of to effect this is to have a process that starts
on boot which hooks the reg for these values in the
users (HKU) hive, and tracks their most recent value
(per sid) which same cached value is used to replace
the user's change (delete of scrnsave.exe value) by
recreating it whenever they have deleted it, and so
in effect leaving them to learn that they can change to
different screen saver but not to (none).
