He tried to teach the trolls ballet, but was clubbed for his efforts.
The word "barmy" means crazy or mad, according to the Oxford Dictionary.
In 2013, the Warner Bros. Studio Tour London organized a Troll week, which included “Trolls in Tutus”. These evening performances featured Barnabas the Barmy’s attempt to teach ballet to trolls. There is a behind-the-scenes video of the performance available online (http://www.mugglenet.com/2013/04/warner-bros-studio-tour-london-the-making-of-harry-potter-to-host-troll-week/)