You need to proceed as I have suggested, and slowly the contact will die off as they get the message.
If not sending cards wil stress you, then send them, but don't put any message in, just sign.
After Christmas, as your contact diminishes, they may talk about you. This is an issue over which yoou have no control, and you should always dismiss anything you cannot affect, because stress in that area is futile and damaging.
If, and it is a big if, what ever information they have affects you in the future, simply tell whom ever brings the issue to you that your ex-friends are mistaken. After all, it's your word against theirs, there is no reason why they should be believed or seen as the sole version of events.
i think once you start withdrawing, the stress you are experiencing will start to diminish, but this is a valuable life lesson. You are in control of your friendships, you don;t have to accept anyone as a friend if you don't wish to - it rather contradicts the concept.