The notion an individual can be truly and permanently banned from any service on the internet is fallacious.
I remember this one BF1942 server. It was the only good one where you could play "Battle of Britain" with a lot of people. They were (are?) super nazi about using expletives. Even using "damn" would get you auto-kicked, and "shit" got you a week's ban if an admin was on. If you typed "s***", it would evade the auto-kick but piss off an admin who would ban you for two weeks for evading the auto-kick. (If I am grateful for one 68kMLA forum policy, it's that we don't have language nannies here. In respect of that, I try to self-regulate my use of expletives. This is in stark contrast to the IRC channel, where language rules are arbitrarily enforced, as if a 12 year old child is going to figure out how to log on to that particular IRC network and channel, and then be scarred for life because he saw the s-word. I avoid the IRC channel and do not feel welcome there for that reason.)
I had to play on there using a number of VPNs, proxies, and "other" CD keys (I purchased the game legitimately though), because I would inevitably type an expletive into the chat for some reason. Maybe it was a day, or a few weeks, but it would happen. I understood the ban on expletives, even if their argument was weak (children playing the game - it's a war game that's rated "T"), but I also thought that being banned for typing "s***" was just plain ridiculous. It even got to the point that they banned two Germans because they were typing in German into the chat and the admin thought they might be using expletives.
I only bothered because I wanted to play that map online and no other Battle of Britain servers ever had any people on them. Everyone gravitated towards this one server for whatever reason, so they had a de-facto monopoly on that map. I even tried to run my own server for a couple of months but nobody would use it.
Before everyone here rattles on about how it's their server and it's their rules and all that other rot - yes, I'm fully aware of that. Go start your own server, you might say - I did that, and it just doesn't work that way. It was a lot easier and cheaper to just find a new way to log into their server and keep playing. I followed all the other rules except for the one about language.
The problem with banning Mibbit is that you are just inconveniencing some legitimate users while causing the malicious users to come up with a new solution - which they inevitably will, without fail, every time, just as I logged onto that server again and again and kept on playing.