If you want records from the lookup be accessible only for certain user roles throughout the system you can turn in Managed by records for the lookup and set up access rights for the lookup records.
If you want to restrict shown values only on a certain page/pages in the dropdown list, you can do the following:
First, in the OnEntityInitialized determine if user belongs to a certain role and set the bool attribute. To determine if the user has a certain role you can see the article below: