I always decline them. "But don't you want three free months of HBO?" they say?

No, because I won't watch it and I know that whether I call in time or not, they will find a way to charge me and say it was my fault for not cancelling. Better to not even have to chance it.

But not calling in doesn't entitle you to a refund. It's just a very expensive lesson (how else do you think a shady company like this can stay in business?)