Asalaamualakum,
I can't help but offer my opinion here. Why does it have to be so bad that someone Christian, or a person simply giving a Christmas card without any religeous meaning behind it, that it needs to be explained to them that we are Muslim and they shouldn't give us a card ever again. That is insane and it is not what we are taught as Muslim's, to accept others for they're own beliefs and hope they accept us for our beliefs. If I sent someone a Eid Mubarak card, do you think they would tell me to never do it again, just cause it's from my religeon? It wouldn't happen. We are too hung up on what is right in Islam and what is really going on in this world. Let's stop imposing and enjoy each other for who we all are and respect each others beliefs. Say thank you for the thoughtful card and don't forget to give them a card and a gift on Eid next year!
I come from a background where I was not religeous but I believed in God, and Christmas was a time to enjoy decorating the house, making good food and pastries, and giving a gift to say thank you for being there all year, and most of all, time to spend together as a family and enjoy friendships, which I have been taught that in Islam is the very same importance, except for decorating and giving gifts(yet people in my Muslim family feel that giving gifts on Dec 26th is Halal)but my values are still directed at what is right about Dec 25th, family and being together. What is most important to Allah subbhanna watallah, is that we do what is right and we take care of our families and pay our zakat and we respect one another. And don't forget to make salat 5 times a day!