Blocking spam

Can someone give me a clue on filtering mail on using the stock sendmail that comes with Solaris? I am unfortunately without a sysadmin right now (anyone need a job?) but I do know enough to monkey with what needs monkeying. Anyway, a certain user decided to dial up (to somewhere else) and use my mail system to send mail to every single AOL user. Today alone I probably got about 600 copies of his bounced mail...never mind that this is deffinately not cool. Anyway, I have the email address that he is using as for the FROM: field so I'd like to simply block that out. Help would be appreciated. Brian