You have a few options:
1. Do nothing. Your most employable staff will leave to escape the abuse.
2. Try to manage the relationship. For instance have all communication go through one person who is empowered to terminate the contract. Set boundaries that if they behave abusively you will terminate the contract. If the client is heavily reliant on you, consider raising your price immediately as you're going to be doing a lot more work than you'd expected.
3. Dump the client immediately. Your staff will thank you but may worry about the company's viability.
4. Keep the client for now (maybe do #2), very actively look for alternate work, and bring the contract to a close once you have new clients.
Whatever you do, express clearly to your staff that this behaviour is not OK, and share what you can about how you are going to shield them from it.
I would rather have no business than one that needs abusive clients. Luckily we’re blessed in this field with many employment options.
Make your lawyer review the paperwork you already have signed, and cancel the contract.
(Alternative: Make your lawyer review the paperwork and raise your price 10 fold.)
Remove him/her.
Under no circumstance would I allow anyone at my company to abuse their power, and abuse other people.
As long as you have very clear evidence of his/her gross miss conduct, then I would terminate your contract.
Money is money, this client didn’t take their money away, right? That’s the only thing that matters. De-escalate the situation and have a nanny comfort the idiot parting with his money - play it smarter, silo off communication from the the rest of the team and have it go through the nanny (and pay the nanny a bonus later for managing the situation). If the current point of contact is not working, have them become the bad cop, and throw in a good cop temporarily. Have bad cop always deliver the obvious requirements, have good cop deliver their special requests. In fact, reset the relationship with the good cop and just have him/her straight up appeal to the client ‘we believe this project can be handled better’.
But don’t send away the money if it’s not walking away on it’s own.
There’s a limit to how much you can and should take. The client can yell and scream all they want about the product. But if they cross that line and turn it into a personal attack, I step in and put an end to it.
1. Protect your people, only let the most resilient people deal with this customer, and only if they are okay with it. In our old company we had an ex-military officer, who would've been capable of dealing with such people and strong language.
2. Try to set boundaries: "This language/those arguments are not helpful. Let's end this meeting/call now and continue tomorrow/next week", then hang up or throw him out.
3. Get the hell out of the contract, ASAP. Never do business with him again.
Have an urgent meeting and discuss your concerns. Make it clear their abuse is not appropriate and will not be tolerated. Tell them you will have no option but to discontinue the project if they continue.