Christmas tree issues...We've had this problem...
with one of our cats, too (we have 3 also). After trying everything with not much success, we tried this rather unorthodox idea, and it's worked quite well.
We got a small squirt gun (the kind you fill with water), and whenever our cat in question (his name's Wally) would either begin messing with the tree (climbing it, messing with the ornaments, lights or branches) we would first sternly tell him NO, and if that didn't work, he'd get a quick squirt, which invariably sends him running in the opposite direction. It has worked very well...Wally gets the message, he doesn't get hurt from the water, but dislikes it enough to stop. Often, all we have had to do is to say NO sternly and just show him the water gun, and he'll quit.
We've had Wally for two and a half years, so this is our third Christmas with him. The first year was a mess...Last year we came up with the squirt gun idea, and this year is much, much better. He has learned what that squirt gun means. It does take repetition and patience. I've heard of other people who have tried this too, and it takes time for the cat to get the idea, but they usually do get the idea...Climbing tree=getting squirted.
If you give this a try, good luck. It has worked quite well for us...And for Wally, too.;)