Sometimes when you try to delete a value in the Reference Codes you get the message that you cannot delete this group because it is still being used by volunteer records.
You cannot delete a Group code value if it is being used elsewhere. You can flush out the records which are using a code value with a filter.
Now you can run a Volunteer Address list, or similar report, using the new filter to find the active and terminated volunteers whose records are using that Group value. After you eliminate their use of the value you will be able to delete the group value.