Pardon my naivety but, couldn't you just consider them out of the company? Write them off the website, stop inviting them to meetings, revoke their access/credentials/...
There is no contract, so what can they do legally?
It sounds like they're mostly disengaged anyway. It might be a good idea to formally "fire" them just as insurance. It might also be a good idea to get the equivalent of a restraining order against an angry ex. Some paperwork that says they will not make any such claims to ownership in the future. But if there is no signed contract, no issued stock, there's nothing to make a claim on. And it sounds like they're too lazy to sue you anyway. :)
Even if you don't formally "write them off" you need to mentally cut them off and move on. It's not worth the emotional effort. Good luck with your business!
The right approach would be a combination of law in your country and your knowledge of Zake character. I would recommend consulting with a startup lawyer and maybe try to find a win-win situation for both of you where he can exit with face, and you can afford it.