..I had the same issue with my two cats. This is the first year I've had a tree with my older cat (year and half) and my kitten is 3 months old. Oddly enough, the kitten started doing it first and the older cat followed suit.
Anyways, I tried the spray bottle, like you did, and it worked for the older cat. We've used it before with him when he was misbehaving so he knew eating the tree was bad. With the kitten though, he didn't get the same message. So, I took a soup can and put a few pennies in it and shook whenever I caught him eating the tree. (Got this from a friend). The noise startles him and soon enough, he stopped doing it. Works too for when the kitten is misbehaving. For whatever reason, the spray bottle didn't phase him.