I know that admin and moderator groups cannot be deleted, but I would like to delete customer and guest. However, when I am logged in as admin and hover over these groups, the "delete" link does not appear. Can it be called manually, i. e. by URL?
The Guest usergroup is not for registered users it represents the unauthorized visitors permissions, there is no way to delete the Guest usergroup, if you delete it your forum users will not see your forum and all functions related to user authorization will stop working.
The Customers Usergroup can be deleted in the database manually. Use phpMyAdmin Database Manager of your hosting service, open the wp_wpforo_usergroups table and delete the Customers usergroup.
Again, if you delete these usergroups your forum will stop working:
These are "must" usergroups.
Thank you very much, I've followed your advice and deleted the Customers usergroup via SQL and left the Guest alone.