How to recognize returning visitors not using cookies?

My coder is working for me on a new game script (mainly coded in php/ajax and javascript).
This is the problem: i do not want people can cheat the game so i need to identify and recognize each single visitor in unique way. Normal cookies are not a solution since they can be easily deleted.

What could i to recognize an user in a stealth way so he cannot easily delete the cookies and play multiple times to my game?

I cannot register the ip in the database because many ISP in my country uses the same ip for multiple users so i would block many users if i ban an ip.

Is there a way to install a stealth cookie not easy to be deleted or something like that?

Maybe flash cookies or java cookies??

Any idea?

thanx