[andix]

What I would do:

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.