if he uses the same exact words every time, just do:

Code:
on *:text:Replace this with what he says:#: {
  msg # .ban $nick
}


if he uses a different sentence but it always has the same word, I would suggest:

Code:
on *:text:*word*:#: {
  msg # .ban $nick
}


the only problem with this is that if it's a common word that a lot of people use in your chat or something, it'll ban them as well, even if they don't mean it offensively. So, maybe add a "regulars" script as well with a check to see if the user is on the regulars list.

Or, get mods that are quick to ban people that are just there to cause problems. I know for some people on twitch it's not that easy, but get a few people that you trust and ask them to help out.