I have obtained a Google Maps API key for my domain.
The examples provided when I obtained my key show the key embedded in request parameters, for example:
Considering that key has to be included in the tags of your HTML pages, to load the JS files/data from google's servers, there is nothing you can do :
Still, it doesn't really matter : if anyone tries to use this key on another domain than yours, they will get a Javascript alert -- which is not nice for ther users.
So :