If you do that on their systems, you are not only liable for damages, you can also do some time in the slammer for computer intrusion and hacking, no matter how trivial you make it to disable the application. This is the worst advice I see on these how do I get them to pay threads. If you want to be able to withhold the product for non-payment it must remain on your systems at all times, it must not be allowed to "go live" if it does you can be liable for loss or revenue even if you own the code. It also has to be stipulated in the contract that ownership of the work only conveys after payment in full and that the work is not to be used in commercial capacity until after final delivery and the conveyance of license. Just going in and turning it of has gotten a lot off developers in a metric-ton of trouble. I have seen it happen to a developer first hand.

Thanks for the info. I have used this method on small scale projects where I provide web hosting, and it worked then. At any rate you're spot-on on having everything in writing before a non-payment should escalate to draconian measures.

Would the DMCA be a relevant tool for things like website design work?

I can not tell if this is an awful joke or a huge misunderstanding of the DMCA?

It's not like I'm a copyright lawyer. If I'm misunderstanding something, enlighten me. If it's that huge and obvious, it won't take much of your time. "That's a stupid question" is a useless response.

If they don't pay up, why can't you assert copyright and block them from having the material hosted? I assume it wouldn't work in all cases, but why wouldn't it help in those where another party is doing the hosting?

IANAL But it seems like this is a straight forward example of contract law. Why try and muddy the waters with copyright law?

In order to claim that you are owed payment for the material the claimant would need to admit that it was a work for hire situation and the nonpaying entity was the actual owner of the intellectual property rights. How can they be in violation of intellectual property law if it is their property?

I have to agree here, contract law is going to be more straight forward than trying to deal with use of the DMCA. You could probably use it after the fact, but you would still be better to argue your case for a verbal contract. You can prove ownership, the business can prove payment if they have made any. The trick on a verbal contract will be proving the meeting of the minds that you had an agreed upon number and that that number was not paid to you. If you receive 50% up front the other party could argue that, that amount was the agreed to amount. In a situation like that it is almost better to not be paid at all, because you can prove that their was no consideration and therefore no meeting of the minds. Remember something in writing, no matter how crappy it is, is better than nothing in writing. Even if it is:

I am going to build an application for X and X will pay me Y the ownership of the software will transfer to X when Y is paid in full.