For what it's worth, pretty much all major credit card companies impose conditions on merchants who want to have delayed or recurring payments, which form the financial basis for this sort of dubious behaviour.

As a result, if someone is trying to pull a fast one like this, you probably don't need heavyweight legal action to defend yourself. The payment authority on your card will have expired when your card did and chances are that the merchant is required by their card processing service to get a new authority for any further payments. If they didn't, and they didn't contact you immediately but let the "debt" run up for a few months, that's basically their problem, and neither the credit card services nor the courts are likely to give them much sympathy. Whoever they sent after you is probably well aware of this, and gave up as soon as they realised the situation, particularly if they're only receiving a percentage of any money recovered as their fee.

I am not a lawyer, this is not legal advice, if you trust any legal comments you read on an Internet forum without independent verification then you're a fool, etc. :-)