It's not the type of game that really need a server to operate. I'm using javascript and html5 right now, and I cant think of a way to prevent the game from being rip off.
Using obsfucator is useless, the game would still work offline.
Implementing a validation scheme is not invincible either. Someone smart can just crack the script and remove the validation part.
Make it attractive for users to play on your site.
For example:
Overall, there are other possibilities to get users to play on your site besides technical restrictions, which - as you already found out - are difficult to deploy in an open source, browser driven environment. But, on the web, this has always been a feature, not a bug.