There are a few things you can do when you receive a junk email:

Read it and decide to spend lots of money with this lovely company!
Ignore it and delete it
Unsubscribe or Remove
Report it as SPAM
Block it

OK the first one is pretty unlikely - I have never felt like buying anything advertised through SPAM and I doubt you have either!

There is a big debate about whether you should reply to or delete yourself from SPAM emails. The argument is that replying or hitting the unsubscribe button just lets the spammers know your email address works and you will get MORE not less SPAM. I would suggest you only follow the remove or unsubscribe link in emails from companies you subscribed to, or if the link is to the website of a reputable company.

If you dislike junk email you can complain to the companies involved: the people whos product is being advertised and the various companies whose systems have been used to send the SPAM. It is quite a tricky process working out who to complain to especially as emails can be forged to make them look like they came from a different location. To read more about this visit Dan Wieser's article. Or use spamcop.net which automates the procedure so that you don't have to know all the details, it also provides excellent SPAM filtering!

Be careful not to report legitmate mailing lists and newsletters as SPAM, unsubscribe from them.

There are many ways you can block SPAM emails, the best way is to use automated filters such as those available at spamcop.net which will filter out a large proportion of SPAM. Most email systems also have filters you can set up to block mail from various addresses etc.