A boycott won't help. You will need at least 10 players to vote on it, for it to have any effect at all. And if a player has their own services corps, they can direct trade and the boycott won't hurt them.
Security Council resolutions to stop nukes, or enforce peace between countries, have no effect. The Security Council can't actually stop anyone doing anything. All they can do is to pass a resolution saying that they don't think a player should do whatever-it-is. The player is free to ignore them.
Your best option is to start a dialogue with the player in question and find out what it will take for them to stop doing whatever they are doing. Then you have the option to accept their terms, or reject them.
Hugs and respect
Jo