If you are in the United States, you may want to consult a lawyer about federal computer intrusion laws before you actually attempt your proposed plan of action. IANAL, however I think unless the client explicitly authorizes you to go into their site, and roll back versions, such acts would constitute "unauthorized" access to a computer system, which in addition to being a felony, has some stiff penalties. Of course, with a properly written contract, you can stipulate implied authorization.

If you're hosting and you set their site to issue 503s for non payment, that's a different matter altogether.