When you catch them getting into the tree, a couple squirts from a water gun will stop that. It won't hurt them, but it will deter them. I know this from experience!
The problem with these kittens is they're fearless, my wife sprayed them once and all they did was look at her like "What?" We bring them to the vets to get shots and they just purr because the vet is holding them, she couldn't hear their heartbeat and had to let them smell hand sanitizer to stop the purring. They are just the happiest bunch.