Wrap it in plastic.
Guard the tree with the Vacuum Monster.
Put the tree behind bars.
Denude the tree of ornaments or branches (although that kinda defeat the purpose of having a tree).
Hang the tree upside down from the ceiling.
Put the tree on top of or inside a piece of furniture.
Or banish the tree outside.
And my favorite: Use an abstraction of a Christmas tree and ornaments instead. LOL