Hopefully you won't need to ban Mibbit or Mibbit clients, but in case you need to, here's a few methods of doing so.
Non-CGI:IRC Connections for CGI:IRC Configured Server
After setting up CGI:IRC, you may want to ban users who are not connecting via CGI:IRC. To do so, G:Line *@ircip*.mibbit.com. Note that doing so will kill all Mibbit users that haven't restarted since CGI:IRC was set up, and will prevent them from joining if you did not set it up properly.
If you didn't set up CGI:IRC, then adding a G:Line for *@ircip*.mibbit.com will block all clients from joining. Be courteous and give a helpful message in the G:Line and tell us so that we can get your server added to the Problem Networks page.
The simplest way of banning all Mibbit users is to ban *!Mibbit@*. This is generally a bad idea.
You may want to keep users out who haven't changed their nick from the default. The ban for that is mib_*!Mibbit@*.