Well, one way to stop a cat from climbing a Christmas tree is to make the tree less appealing to them. You could try covering the tree with tinsel, ornaments, and lights, so that it looks too sparkly and shiny for a cat to want to climb it. You could also place a few deterrents on the tree, such as aluminum foil or sticky tape, which will make it uncomfortable for the cat to touch. You could also try placing a fence around the tree to make it more difficult for the cat to access it.