It's probably not a bot, because he's changing up the text and getting past the other anti-bot measures. I think there might already be a link ban, because the last few messages haven't tried to link and have just had the URL in plain text.
Is there a way to prevent particular strings from being allowed in posts? There'd probably need to be some magical regex involved, too, to account for the munging that he seems to be doing lately. Or something that would cause posts to be auto flagged?
Is it even practical to do so?