Claim you were a Christmas tree in your former life. If s/he tries to bring one into the room, scream bloody murder and thrash on the floor.
Go to the mall with your roommate and sit on Santa's lap. Refuse to get off.
Wear a Santa suit all the time. Deny you're wearing it.
Sit in a corner in the fetal position rocking back and forth chanting, "Santa Claus is coming to town, Santa Claus is coming to town..."
Hang mistle-toe in the doorway. When your roommate enters or leaves the room, plant a wet one on his/her lips.
Hang a stocking with your roommates name on it. Collect coal and sharp objects in it. If s/he asks, say, "You've been very naughty this year."
Paint your nose red and wear antlers. Constantly complain about how you never get to join in any reindeer games.
Make conversation out of Christmas Carols. I.E. "You know, I saw mommy kissing Santa Claus underneath the mistle-toe last night."
Wrap yourself in Christmas lights and roll around in the snow.
Sing: "All I want for Christmas is my roommate's two front teeth..."
Give your roommate the gifts from the twelve days of Christmas song.
Build a snowperson with your roommate and place a hat on its head. When it doesn't come to life, cry hysterically shouting "It didn't work!"
Whip your roommate, screaming, "Now Dasher, now Dancer, now Donner, and Blitzen..."
Tear down all your roommate's Christmas decorations yelling, "Bah Humbug!"
Wake up every morning screaming "Ghost of Christmas Future, please have mercy on my soul!"
Tell your roommate you're moving out. Santa's buying you a house on 34th Street.
Pin a pointsetta to your lapel.
Make anatomically correct gingerbread people and eat the best parts first.
Put on a fake white beard and insist that all your roommate's friends "give it a yank."
Ring jingle bells maniacally saying, "Every time a bell rings, an angel gets his wings."
Stand in front of the mirror reciting "How the Grinch Stole Christmas" over and over in your underwear.
Smoke mistle-toe. Do what comes naturally.
Watch your roommate when s/he is sleeping. When s/he wakes up, sing, "He sees you when you're sleeping..."
Steal a life-size nativity scene and display it in your room. When your roommate asks, tell him/her "I had to let them stay here, there's no room at the inn."
When your roommate goes to the bathroom, rearrange his/her possessions. Tell him/her that Santa's elves must have done it.