So it finally happened to me, guess I should be grateful it's the first time!
Final website delivered, they claim some work wasn't done (it was), and rather than communicating that and asking to fix they simply cut off all access to site (it had gone live) and bring in new developers (aledgedly, nothing changed).
Problem for them, my business name is all over the code, and my contract states very clearly ownership is only handed over upon payment of final invoice.
So I created a document detailing & providing proof of my work on the website, and my copyright ownership, and sent to their web host (which is mine too, I had recommended them to the client). After the host communicated this to them, it seems they have now decided to pay the outstanding invoice amount.
So one for the good guys... (okay, the "okay" guys...)
From their initial reply to the web host it was very obvious they had zero understanding of internationally recognised copyright laws around authorship & ownership, so this was the key thing that had them pay in the end I believe.
Side note, I took this over from a previous designer/developer that had created a pretty bad site for them, but their design work had included the logo this business is now using everywhere. And she didn't pay these guys for work they had done either, so she probably doesn't own copyright on the logo either...
Mainly because prior to this instance, over 20 years in the business (not all working for myself obviously), this is the first time this has EVER happened... plus there are some projects where the site goes live before all items are finished, and it is very difficult asking someone to pay a final invoice when there is still outstanding work to be done.
But once bitten...
For new clients now my invoicing terms will change from 50/50 to 50/30/20, with the second 30% invoice due prior to a website going live.