netmeg - 3:40 pm on Dec 7, 2009 (gmt 0)

Remove the search box. It'll cost you money, but that's better than being banned. In fact, I'd remove any and all monetization from your site in the short term, because that person will likely be angry when the search is gone.

Gather up your server logs and see if you can identify IP numbers. If it's just one IP, or a block of IP numbers, maybe you can block them at the server level. If it's a bunch of them, you probably can't. (They could also be coming through proxy servers)

Send the whole shmear to AdSense support for investigation. Be prepared to lose that extra revenue.