Any business can refuse to serve any person for any reason they wish (unless it's one of the "protected" statuses, race, religion, sexuality etc).
Despite you being in the right in the past, the company doesn't want your business and refuses to deal with you.
It may not feel right, but it's part of being in a free world. You have the choice of who to buy from, they have the choice of who to sell to.
As long as no contract or agreement has been signed you're out of luck, just go somewhere else.
As business goes losing such a Sale
doesn't make much sense, but you're hardly restricted in choice, so go to another, just don't tell the sales guy this story until AFTER the agreements are signed.