I Think you have missed the point entirely...

We already fired one client. That meant I had to fire some staff, because without clients, no staff...

The point is with everything about this client, would it be an overreaction to fire them, and what are some ideas to not have to take this drastic step with long-term clients.

Payment is solved, they ask for services, we can use their retainer to quote them, if they don't pay, we don't do the work (seems heavy handed to me, but I am an idealist that would just like for others to follow through on what they say, and agree to half-as often as I do, and create an environment where I receive a well thought out list of wants with a budget.).

To be honest When people do this, I usually just look at the budget to see if it's so low I have to reject pre-costing. The cost will be the cost will be the cost, but it's nice to have an idea. Most of our clients pay much more now than they ever did before, but they also get much better results now!

This isn't a one client issue. When we were smaller, no such issues. Now we are growing, thanks to these long-term clients stories and case-studies, they decide they can throw weight around, and quite honestly, it's not on, so rather than replace all clients I look for solutions... Sorry if this seems to fly against your solution, but it seems you have very fixed notions, and an active imagination.

I think he's an asshole in those matters

is not

I think he's an asshole.

There is a gross difference between thinking anyone that does not pay what they owe is being an asshole when even raising issue of not paying, and just overall concluding that they are an asshole. The two ideas are a world apart. Maybe it's linguistic, but you definitely got the wrong end of the stick there