Hi Byron,
In the group category custom field for group type, you can select the default option for this custom field(http://take.ms/vqmjK).
As for not displaying the other options, perhaps you can just hide the field entirely since you do not want users to select other options.
You can use the following custom css do hide this field in groups.

#es.view-groups [data-element="type"] {

	display:none;

}

Great suggestion! Thanks again Raymond.

You're most welcome Byron.
Regards.