Excellent question!
Unfortunately, as far as linguistic analysis goes, I can't offer any better solution to those above. However it would be possible to give his ISP two IP numbers along with timestamps and ask if they're the same person. You could explain the reason and point out that you're not asking who the person is, only if they're the same person. The might - they might not.

In order to stop it happening again (without going through the ISP), you could try the cookie idea .. but I'm not too sure it will work. If he really wanted to, he could find the cookie.

Instead, you might consider that if he is so desparate to be a part of the community, he'd should be willing to reform. Thus I'd suggest emailing him and letting him think you've done some sort of analysis and thus you "know" its him. Tell him why he was banned in the first place and that he's on a very short leash. If this annoys him to the point where he leaves, then is it any loss? If he re-offends then consider just banning his ISP. If he switches ISP just to get into your community, then let me know what your community is: coz I figure if it's that good, then I want to be in it too!

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