Yes there is... Only, you'll have to mess a little with the select screen. You have to make sure you can't reach some of the icons on your screen. Then, put your boss characters at those spots and you won't be able to select them. It's generally best to make sure you also won't be able to see the small select icons. You can do this by either making the icon appear outside of the screen, or by moving sprite 9000,0 to a different place.