Yeah, PyroElf got it. Cancel your Content ID's. You can still use your characters until this month is up (you already paid in advance for them), but you won't be able to use them on the first.
I believe they give you a grace period... I don't recall how long that is, though. I've heard anything from three months to a year before they delete your characters once the ID's are cancelled. I've also heard that they now NEVER delete your characters. So, I don't know. Maybe try to call SE on that. But I would probably let it go a couple months, then sign up for a month, then cancel again, just to be sure.