Thanks for your comments, everyone.

In terms of forgiveness, banning, etc., we didn't ban him for ever, just for a few weeks. That's our policy, like a cooling-off thing. We can't set custom cookies, it's too late for that, and the ISP he uses is huge. Believe me, we've considered those tech solutions.

I'll check out the modules mentioned by mojotoad, but in the meantime I found Text::Document -- it will compute a JaccardSimilarity and a CosineSimilarity between two strings, plus giving word frequencies, but I don't know anything about what those terms mean. Can anyone enlighten me? If you compare two strings which are identical, they come out as 1. If there's no match at all, it's zero. But in the middle..?
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“Every bit of code is either naturally related to the problem at hand, or else it's an accidental side effect of the fact that you happened to solve the problem using a digital computer.” M-J D