If you're turning customers away, it probably means you are not charging enough. Commercial oven and kitchen cleaning is quite a specialist area. Customers would probably want their whole kitchen cleaned and degreased (including the hood and ducting), rather than just the oven. You will need extensive training and equipment (and a team, because no food establishment will want their kitchen shut for very long), this might cost thousands. If you're just starting out, it might not be the easiest expansion route.