To stop a cat from climbing your Christmas tree, you can try using deterrents like aluminum foil or citrus scents, providing alternative climbing surfaces, or using a cat-proof tree skirt. You can also try giving your cat plenty of exercise and mental stimulation to help reduce boredom and stress. Lastly, you can provide positive reinforcement when your cat avoids the tree, and give them attention and affection when they play with other toys instead of the tree.