Sprinkle ordinary ground pepper on the skirt or base of the tree and on the lower branches, and your cat will sneeze and leave! It’s harmless and won't affect your cat or damage decorations.
There are also sprays, such as bitter aloe, sold in both pet stores and pharmacies. Spray a little on the lower branches and chances are your cat will be discouraged from returning. Sticky sheets usually used to deter scratching on furniture will also work when placed around the base of the tree. Also, consider putting any special gifts for your cat out at the very last moment, especially if catnip is involved. The feline nose knows.