Two places, with different safeguards:

• On the Menu Groups table, click the trash button in the row and confirm "Are you sure you want to delete this Menu Group?". This path does not block deleting group ID 1.
• Inside the builder, use the Delete (trash) icon in the Current menu card; the Delete menu group dialog warns that the group and all of its items are removed permanently. In the current version, this path specifically refuses to delete the group with ID 1 ("Cannot delete Menu Group ID = 1").

Either way, once deleted the group disappears from the list and, if it was active, stops rendering on the site. Because the ID-1 protection only applies to the in-builder Delete button, keep menu group #1 as your base group and avoid deleting it from the table view.