Sigh. I feel for web designers who don't get paid, but this is not the appropriate response. It puts you potentially in legal danger and loses you both this customer and anyone they ever talk to.

If you failed to pay a contractor building you a house, they can take a lein against the house or sue you, but it would be quite illegal for them to start tearing the house down. Similarly, unless you have a written contract saying that their website is merely a service you provide, defacing their property is not putting you in a stronger position, morally or legally.