Most ISP's now have what are called spam filters. You can use them to block the download of spam or filter the spam out of your inbox. In addition most mail clients have filters on them as well. You can use those from your end to automatically move the email to the trash box. The problem though is you end up still downloading the messages with mail client filters.

A few other sugguestions, is to get a "dummy" email account through one of the free email providers. Use that account's email for all the registrations you do online. That will help to cut down on the spam to your primary account.

Another is to be sure to read the privacy policy. Unless you just have to for business or personal reasons, i've always recommended boycotting sites that sell or distribute contact information to third parties. Also when you registar be sure you do not have checked "recieve further email" unless you want it.

People can get your email from an assorted number of places. I think in some cases it can be as simple as you've left a message on someone's guestbook and another Users comes back in and "steals" the address of that guestbook.

hope this helps

Artimis