Ah but you miss the beauty of it: He'll think that a moderator is deleting his topics

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I'm sure they tried that, I'm betting he used proxies. I would have kept up at it, using simply sql queries to delete the spam rather than doing it all manually. Eventually, he will grow bored of it... if not, tell him how easy it is to delete his spam so he knows all his work is gone in .01 seconds.
New email addresses aren't hard to get.