Junk mail return?

When you get junk mail and send it back with a reply you get Administer mail return which never made it to the sender so why can't we do the same and Bounce our email back to the sender the second time they send the junk mail? Going into the junk email box means we still get the crap so why can't we stop it at the server?

My ISP (Covad) offers various levels of spam filtering. If I choose, I can have the spam go into a quarantined inbox that I can view in my mail program using IMAP, and then use "delete" and "purge" in the program to delete the unwanted spam from the server w/o downloading.