We have two cats.
One is coming up to 1.5 years, so basically coming out of kitten phase. If she goes near the tree she gets sprayed with water and a firm "No!" shouted at her. Now she's either very weary of going near the tree or if she does, we shout "No!" and she walks away.
Our 4.5 year old is a different matter. Since getting neutered (used to belong to our neighbour) he hates going outside, so he stays in at night. I came down stairs this morning and had to retrieve 14 Christmas tree balls from behind our sofa's! The other morning came down to find him lying on a strand of tinsel he pulled down.
He's very clever. He won't pull down the Christmas tree balls whilst me and my wife are up and about. He waits for us to go to bed before he has his fun, so its impossible to discipline the swine with water spray.