you can always apply rules to mails containing certain words or coming from certain adresses ... but the best spampreventer of them all is to bounce stuff back...
At least until spammers catch onto some version string or formatting that Mail.app includes. Then they'll know you actually opened the mail, which will bring even more spam down upon you.
yep, that's mail.app...
probably outlook express will have it when the os x version comes out, microsoft seem to be in favour of 'adopting' mac features ...