To remove mail merge groups entirely from the system they will need to be deleted by following the steps below:
Go to Setup and click Roles & Permissions.
Click one of the roles that is assigned to you.
Click Role Permissions and then Edit Permissions.
Search for the name of the mail merge group you need to delete.
Enable the view permission for this group by clicking the eye icon so that it turns green.
Click Save.
Go to Setup and select Content.
Click Mail Merge and click the folder in which the group is created.
Click Edit next to it.
Select Delete.
Once the above has been completed, the group will not be displayed as an option when adding a mail merge letter to employees.