You are an officer and a tank(?). He is a veteran to the game and the guild, and a dps. You outrank him. You need to take the authority and let him know that the problems are going to stop immediately. You've lost essentially a 10 man team due to him. And the needs of the guild > the needs of one player. He is obviously not helping the guild grow, he is stunting the growth. I say tell him to keep it to himself, or share his gifts with another guild. Dps are easily replaceable. Personalities aren't, but I'm not sure his current charms would be missed.