To prevent cats from climbing a Christmas tree, you can try placing a few double-sided tape strips around the base of the tree. Cats don't like the feel of the tape on their paws and will avoid the tree. You can also use aluminum foil on the lower branches of the tree, as cats usually don't like the sound or texture of it. Additionally, you can use motion-activated air canisters, which emit a loud hissing noise when triggered, to discourage cats from approaching the tree. Finally, provide alternative climbing options, like scratching posts or cat trees, to give your cats something else to climb.