This might sound mean, but it's not harmful and it actually works. It's a common trick used for training cats (yes, you CAN train a cat).
1.) Fill a spray bottle with water 3/4 of the way up
2.) Choose a pleasant and distinctive scent to add to the bottle that is NOT already used in your home. You can use lavander oil, lemon juice/zest, mint extract, rose oil, anything. Use this to fill the rest of the bottle (or add just a few drops if its a strong scent)
3.) Mix it around. Now, go up to your cats and spray them directly in the face with it. Point blank. It won't hurt them.... but they'll definitely be displeased. Spray twice if you get the option
4.) Now spray all over the tree and around the base. You can also spray wherever else you don't want them to get into... like the bathroom garbage.
**Though this may be hard to do, it really doesn't hurt them and it'll work! They'll associate the scent with the negative experience of being sprayed and will not be likely to go near a tree that smells like the scent. And don't worry, they'll forgive you.