Hearing stories of ticket inspetors remind me of a few years ago. I was charged £95 for a train ticket after being told I could use my return on an earlier train (by somebody in the ticket office). This was apparently not the case! Due to it being a quick trip I had no ID or cards so he threatened to escort me to a police station, I persuaded him otherwise and he just took my details. I complained to the train manager who agreed that it was ridiculous, he said he had sorted it but he actually hadn't. I get the fine and refuse to pay, end up paying half of it after being threatened with legal action. Moral of the story is, all train conductors are absolute piles of something nasty!
On the other side of things, what he did was very harsh but it was actually within his rights. It doesn't help you out, but in this case he followed correct rules that allow sellers to legally destroy cards under good suspicion of fraud. I think you got hard done by, but if the card did say void then you should probably have got it replaced.