This is the same thing as the junk-mail that comes to your house. Anybody who gets your name can send you stuff and put you to the 'sales-test'. Some ways to avoid internet junk-mail are: 

1. Don't give out your email address
   - this isn't totally practical because we want people that we like to be able to reach us. But we could point out that there are a lot times on the internet when a web site will ask for our email address. We can be just a bit more careful of where we place it. A lot of the time people ask if they can put you on there mailing list. But not always. A key: vigilance and choice.
2. Delete a lot of mail from your INBOX
   - I suppose this is akin to throwing away all of your "no-interest till next year" credit card offers that you receive on a weekly basis. 
   If you are on a network like A?L it can get ridiculous. TONS of useless mail EVERYDAY. It seems like they track you on a network and send you email from every site you travel through or something. Maybe that sounds paranoid, but if I don't sign up for ANYTHING or give out my email address to ANYONE and then the following weeks receive TONS of email from sites that have topics related to the sites I was just visiting. Well? What is that?! Maybe it's A?L tracking control.
3. Filter your incoming mail.
   - there are different rules that we can apply to incoming mail. Each email method varies slightly. It is basically broken down into BLOCK and ALLOW. It's Boolean stuff again. Which end of the stick do we want to come from? Do we want to block certain people out? Or, only allow certain people in? Maybe a little of both.