You seem to be acting out of anger rather than considering the most sensible course of action, which is to briefly report your unsatisfactory experience with this client to whichever site he found you on, and then move on with your life.
While you may gain some small measure of satisfaction from damaging their reputation or trashing their software product, ...
You've answered your own question. There are so many red flags here that you could host a parade.
The spec isn't even close to being finalised: Not a (huge) problem per se, but they appear to have no idea what they want and are looking for you to tell
them what they want. That's not the way a client/developer
relationship should work.
Endless meetings: See ...