I tried everything last year. I put tin foil all around the bottom, I sprayed it with citronella, I made citrus 'baubles' out of orange slices and grapefruit slices dried in the oven and hung them all around the bottom... nothing worked. Then we put a large dog crate on it's side around the tree, basically caging the tree into a corner... they jumped over the cage and still destroyed the tree. In the end I redecorated the entire thing on Christmas morning before the guests arrived (by which time they'd snapped it in the middle and it was being held together by the inner tube from a roll of wrapping paper and copious amounts of tape. Even then it was balanced precariously, couldn't be touched or even breathed near, and bent over at the top like Mt Crumpet). It was thrown away after Christmas.
I'm determined to have a tree. This year I've bought a cheap tree (thanks Tesco) and will be treating it as a disposable purchase if they break it again. I'm also not putting any sort of garland on it because if they grab one end and pull then it screws up the whole thing. I'm hoping that the fact my older cat is now 2 and my younger cat is nearly 2 means they will have grown out of the destructive kitten stage. I'm aware they will probably ruin it again and I'll end up redecorating it several times. If you find anything that works, please post it!