A bully is the opposite of a champion, and goes too far in pushing people to use the wiki. A good champion knows how to lead people in adopting the wiki, while a bully might get upset at someone for emailing rather than using the wiki.
If someone reacts really strongly when you use email or another communication method other than the wiki, you might have a bully on your hands. Also, a bully may overemphasize the wiki in group meetings, and chastise people for not using it.
Talk to the bully and help them to understand the fine line between being enthusiastic and bullying people. Some people don't realize they're bullying, and just need guidance on community etiquette.