At a certain point the answer is just lawyers or at least the threat thereof to kick this into an actual problem for the client rather than a passive issue. Hard to say if it's worth it without knowing what other steps have been taken or how critical this client is to the business but if it's becoming a financial risk it's probably about time to escalate this to a level beyond a passive aggressive blog post.

Even without specific clauses there are legal remedies for breach of contract you just have to be willing/able to use them.