Your cat is probably curious about the tree, so it's understandable that they would want to explore it. The best way to keep your cat off the tree is to make it less interesting. Try spraying some cat repellent on the tree, or using aluminum foil to make the branches less appealing to your cat. You could also try placing your tree in a different spot in your home, such as a room that your cat doesn't have access to. If all else fails, you could try blocking off the tree with some kind of barrier, like a gate or a screen.