I have been lucky so far... Luka understands "No" very well. It took him maybe, one day to learn. All I have to say is one "No" and he instantly stops, even with distractions. I took him with me to get the tree and we brought it home. I let him sniff it when I got it in place. He went for it once when i was decorating it, I told him "No"... after that, he's left it alone. At 15 weeks, he's a VERY good puppy.
I hope it stays this way. However, I won't leave him unattended with it.