Is this legal? I assume if they wanted to (which admittedly is unlikely) they could send you to collections for not paying for the renewal that you agreed to when you subscribed.

In short, no it's not legal. Just because they can't take the money from you doesn't mean you don't have to pay them.

The subscriptions are fixed terms. Not lifetime. Why should they be allowed to bill me beyond the subscription term?

That depends, if the subscription is until canceled, then yes, it's a lifetime term.

You’re subscribing for a year or whatever is the fixed period. How is it illegal to pay for it with a physical check?

It's not illegal to pay for it with a physical check; it's illegal (unless your original agreement automatically terminated after a year) to not then pay for the automatic renewal at the end of the year.

Happy to break the law in self-defense.

At least you admit now that it actually is breaking the law.