IRC is not associated with Blockland. The Add-On, RTB, connects to IRC and tells IRC that you want to use this name and information. Anyone can go in IRC, set their name to yours, and talk because this service does not care to know that the name is also used by someone on an obscure game. Nothing was hacked, you never had exclusive rights to using that name there. RTB4 has it's own chat system which won't even bring up such a problem, so just wait for that.