The legal system is a realistic option only if there was a written contract in place, which is not always the case.
Intellectual property rights are all the rage these days and that would mix into this.
That said, there are a ton of things having to do with the scope of work, reviews of work in progress, acceptance or rejection of completed work, and payment terms that are absolutly the contractor's responsibility. But again, securing clear title to the work is the buyer's concern, not the seller's.
I would love to see a single case ever where a company's domain and website were turned over to a contract designer due to a pay dispute.
The seller is not "limiting access". They are not taking away the work they produced. They are instead publishing defamatory messages on the client's domain against the client's will. They are essentially claiming the domain as their own, which is the real issue here. This wouldn't be an interesting or controversial story if nycfreshmarket.com was a white screen, or a coming soon type thing.