I did this for our new dog and toddler, and discovered that we weren't waking up to the "bang, jingle, jingle, jingle" everynight after we put up the Christmas tree (ours tend to stick to the jingle bells, and that's fine with us, lol).
I bought some fine (small holed) dark green netting and wrapped the tree in it, leaving enough to gather at the bottom, to keep the cats out of the inside of the tree and from drinking the water. We also would tie it up, but the last few years we have just gotten a smaller tree (we have a very big, metal, stable tree stand). I have wrapped the tree for the last 3 years, and it has worked like a charm!
The best part is, if they do still manage to swat something off, it is caught in the bottom of the netting, instead of hitting the floor
I stapled it together on the back and underside of the tree, and just remove the staples when it's time to take the tree down.
Here's a pic of last years. If you look close, you can see the netting: