In ProSelect, an order group can be deleted only if there are no orders for that group.
To delete an Order Group, open the Order Group window (under the Order Menu) and click the Delete button.

Hint:

To delete orders for a Group quickly, select each order item in the Place Order Dialog and hold the Alt/Option key down while you click on the Delete button (at the bottom). This will do an immediate delete of orders without asking you for confirmation.

If you wish to remove most or all of your Orders and Order Groups, you can use the "Clear All Orders" (from the Orders menu) and recreate your Order Groups and re-order the ones you wish to keep.

Bug found in v4.0 - 4.0.8

If any orders are missing or changed in any higher Order Groups than the one deleted, then please see the article "Orders missing or changed after deleting an Order Group". This has been fixed in newer releases.

* It is not possible to delete an Order Group in ProSelect v3 or older.