Actually, nobody knows anything except that onemine.com is willing to hijack the websites of its clients. Everything else is you simply accepting their side of the story without question.
Indeed, having worked in related fields for the better part of two decades, I cannot think of a single example where a designer attempted to shake down a client with a bogus claim of non-payment. But I can think of an astonishingly depressing number of cases where clients wanting something for nothing decided that stiffing their vendors was an easy and acceptable way to increase their own margins.
All other considerations aside, if the client didn't pay, the work isn't theirs. By law. Full stop. And if the client was stupid enough to host their site with a vendor they're shafting, the the vendor is well within their rights to limit access to that site in the fashion they did.