I remember trying to discourage my cat from eating christmas tree, i read somewhere that cats hate pepper, so i piled up quite a lot of it and then i had to stop him from eating christmas tree AND pepper.
Sounds like one of my cats, he will chew on literally anything.
The one thing I've found that works with all of my cats and my dog is lemon oil - they can't stand the stuff. I make a spray out of 70% rubbing alcohol and lemon oil (like 95:5 by volume) and spritz it around the tree every few days, they won't even come close.
Good point, looks like no essential oils are safe for cats. Mine will certainly not ingest it (or even come near it), but safer alternatives would be better. Can probably accomplish the same thing with some dried lemons or lemon zest.