Wilco,
As Kevin says, there's no real way to "force" them to press either button.
They could;
a) close the browser
b) just ignore the page (which at the very least would cause the session to timeout eventually)
c) type a different address in their Address bar of their browser
d) select something from their favorites list,
and so on.
So whatever you do, you'll never "force" them to press a button.
So instead of hiding menu items (which is possible) perhaps there's some other approach you can take. Tell us more about why it's so important for them to press Save or Cancel and perhaps we can offer some advice on another way to do it.
Cheers
Bruce