Possibly the best way that I have thought of would be creating a very own API key to check and see if that key is active. Developers will be able to enable/disable the keys in order for the site to either work or not work. This would take quite a lot of work to build and make though. There are some other ways that I have thought of but if the client is smart enough, they can still get around it. Still working on some ideas and what not for it 
Best regards,
Michael // 1amDev
Best regards,
Michael // 1amDev
