Maintaining a polite silence with a know it all is no longer possible. It's time to tell this arrogant individual exactly what you think of their attitude. Don't be surprised when other people in the group shower you with thanks. They're also tired of listening to this blowhard, but were too timid to say anything. It's possible you will be dismissed from an influential position. This won't seem like a big loss. Greener pastures are beckoning; go and explore them.