If you really don't want to participate anymore, maybe you just need to make it clear like windchime said and then stick to your guns by not buying anything. He'll get the idea when he doesn't receive anything.
The tricky bit is that keeping communication channels open and the only thing I can suggest is that you send a warm card wishing them all well and then try to keep regular communication.
It's important to remember thought that you are not responsible for your fathers behavior. I could be way off here (and if I am I'm really sorry) but it sounds like your father relies on petulance to make you give in to him so he might kick up a fuss to begin with but hopefully if you don't give in he'll realize that wont work anymore.
As long as you are polite, explain it's not a rejection of him and are open towards him, if he decides to not respect your decision then that's his decision. You are an adult after all and your entitled to make your own decisions just as much as he is. It sounds harsh but he sounds like he's been pretty harsh to you too and that's not fair either.