1. Put it on top of a piece of furniture, or use something as a base.
2. Use a pet gate.
3. Leave lower branches not decorated.
4. Attach the tree to the ceiling.
5. Take out lower branches (though it is a questionable look from design point of view)
6. If you are too desperate, just put it in a cage.
7. Or wrap in plastic.
Keep your Christmas tree safe and have happy holidays! :)