It's their business and they have the discretion as to who they do and do not want to sell to. Apparently, they do not want to sell to you, so you might as well find another shop. I can't really say that I blame them, because they are just covering their own butts. If I owned the shop, I can't say for certain that I wouldn't do the same thing, whether you like it or not. Apparently you made some mistakes in the past, and whether you've changed or not, you need to live with it.
Welcome to the forum.