Hi Gary,
Unfortunately in your instance there is not a specific UI component to toggle this option off and on for users. It is built in to allow for further Sorting and Grouping. However, it should only be populating that dropdown that appears with fields that the specific user has field access to. Any field where they are set to permissions of none should not appear in that list. This does mean they might see fields they have access to but aren't on the typical forms and reports they can use but they will not see the name of fields they have been given permissions of none for. I hope this information is helpful Gary.
