You should anchor the tree to the wall with a couple of loops of fishing line and an eyebolt in the wall a little above the halfway-height level. That will keep them from tipping it.
No breakable ornaments on the bottom 3 feet of tree, and none that are within swatting range if the animal is perching on furniture, either. Fine broken glass and cats do not mix -- they step on the shards and then ingest them when they lick their cut feet. If you use breakable ornaments at all, be sure to firmly wire them to the branch.
Also, be sure NOT to use tinsel icicles anywhere on the tree. Cats will often eat this stuff, and it gets tangled in their intestines. Surgery to try to save them when this happens is usually a major and often futile ordeal; eating icicles is nearly always fatal to cats.
As to keeping them away from it, squirt guns are usually good. If you have to leave them alone when they have access to the tree, you can spray the tree with a cat-off solution at regular intervals. Also, using a wide skirt with aluminum foil under it might help -- cats can't stand the sensation of walking on foil.