Not much you can do I'm afraid. Channel bans have nothing to do with the mIRC client. The ban may not be aimed at you, but rather at your ISP due to other users of your ISP being persistently abusive.
You need to contact the channel ops (Try typing /mode #channel b and see who set the ban on you, or who has set other bans to get their nicknames), and message them when they are online. However, it's quite possible that you won't be unbanned, in which case, there isn't anything you can do about it except go to one of the other hundreds of thousands of channels!
G'luck.
Regards,