I'm going to ruin a board game for my friend's family at our next gathering
Why do you want to ruin the board game?
They should know better because I told them about it before hand.
Ruining friends property on purpose, even if you gave your friend warning, isn't the nicest thing to do. If you have an issue with that game in particular, talk to your friend and find something else that interests everyone.
I'm going to do it anyway even if I get in trouble.
I really think you should reconsider this action, what is your purpose for wanting to destroy? How are they going to feel, even if they knew you could do something like that? I think it's best you just join in their game, and do your best to "destroy" them by winning hard!