There's probably ways to do it by using CSS to overlay a cover element over half the page.
Does the client have access to the site? By that, I mean can they get the database and files that make up the site? If they can, then you've done it wrong in the first place, and they can pretty much run of fwith it, and it'll be up to you to start some sort of legal proceedings to get the rest of the money that's owed to you.
If they don't have access to the site, then make sure that you tell them that they will get what they have ordered form you just as soon as they pay the entire bill, and then don't back down from them.
Of course, this all assumes that you have some sort of contract or agreement about the price of the site and what you are going to provide for that money in writing... You do have that, don't you?