That's perfectly solved by a "no loading" orders. In general it's a good practice to have "unload all no loading" on all your one-way trains. But even if you accidentally started acceptance already it will stop after a while once you add no loading order. And cargo rating doesn't even matter since you're not going to pick up that cargo anyway.
No it's not. If an existing station serving IndustryA is expanded and with its expansion includes IndustryB (which supplies raw materials to IndustryA), as soon as a train with a TypeB car enters the station, even if it has a "no-loading" order, the station starts accumulating materials from IndustryB.
Even if what you say was
happening, if by mistake you forgot to place "no-loading" on a single train, there would be no way to make the station not receive cargo of that type, except if you loaded an old save.
EDIT: The problem in this case is that since the station receives typeB cargo and one of your trains contains such a car, the rating is
counted, you have no way to avoid it.