Q. Can I enforce a Group Policy preference?

A. While a Group Policy preference can't be enforced in the traditional Group Policy Settings way, where the user interface to change a value is disabled, it's possible to configure Group Policy preferences to re-apply themselves at each Group Policy refresh. You can set a configuration using Group Policy preferences and the user can change it, but after the next Group Policy refresh, the preference will be applied again.

This is actually the default behavior. To make a preference only apply once, you we have to select Apply once and do not reapply in the Common tab of the preferences configuration, as shown here.