Since the more recent versions of formmail.pl are mostly useful to spammers looking to mask their identity, you might notify the ISP of origin. If they get enough complaints this will be one of a series of pulled accounts for the spammer. Other than that, the best thing to do is let this go to 404, why waste your time and bandwidth on trying to fight it? The only way to actually win such a fight would be to root their box and cause them misery-- but since that's illegal (even in self-defense) I strongly recommend against it. So here's what I'd do: submit a polite report to abuse@ISP.com and shrug it off-- I suppose I'd archive that portion of the logs, too. Just like I would for any other suspicious set of error messages.