You guys need to spend some time learning how to moderate IRC better. There are many different types of bans: you can ban a certain nick, you can ban a host, or you can ban a channel.
If 5 people from the same channel are spamming the lobby, use a channel ban. If somebody with the same nick keeps coming in over and over again despite banning them, ban their nick.
You can also set modes "m" or "i" on the channel. "m" will prevent anybody without voice from talking, and "i" will prevent people from joining the channel without an invite. These are temporary fixes.
Read this for more info:
http://wiki.swiftirc.net/index.php?title=Ban_guideAs for the ban list, clear out the ones from the bottom first if there is no room.