The other day i was really thinking about how to prevent your plugin to be on warez website.
I know everything can be reverse engineered. But a while back I did something with an app and it really worked out great.
I added bunch of checks to my app. And it checked everytime if the app was allowed to install on that uid.
Is it allowed to add the url of the page with the license id to a DB ? When someone activates it. It goes to your DB with the url. The next time if he wants to activate and the id is already used or not available in your DB. It will not work.