procmail can block it with a regular expression, and if procmail isn't appropriate (like in an environment where you are forwarding messages via sendmail's virtusertable) then sendmail can be hacked to do it via perl. as an admin, i'd prefer not to filter anyone's e-mail, but after SirCam we pretty much had to. the SirCam filter blocked 100's of gigs of SirCam infected e-mail - that's 100's of gigs people didn't have to download via POP.
long story short, find out if your host can help out, or at least if you can use procmail and DIY.