I wouldn't change anyone's website to say they haven't paid a bill. It won't let you collect any easier and you might get sued and lose.
At some point you have to say no more work until we get settled up to this point.
At some point you have to say no more work until we get settled up to this point.
EDIT: Having the "finished" site on a test host (or even just a temporary DNS name) until the final payment seems like a shrewd move. The work is complete, but if you don't get final payment it doesn't go live.