You cannot stop receiving them but Outlook has a Junk Mail facility which can filter out junk addressed. Check your Outlook help for details of how to switch it on.
If you do receive unsolicited mail then do NOT reply to it - even when it says something like "reply to this message with REMOVE in the subject line" - nine times out of ten this is a ploy to confirm that the email account the sender is spamming is active.
I received at least part of the solution when it was far too late. Getting a secondary account to use for message boards, competitions, newsletters etc only and use your premier account for family and close friends.
I've put into place Message Rules and all I have to do now is Delete en masse. There will always be some that get through - but this should help.
I'd recommend a utility such as Mailwasher. With this you can preview all emails before downloading (good way to spot a virus before downloading) and delete or bounce those you don't want. The latter feature is especially helpful as "bouncing" sends a message back to the originator that the address is not valid/in use. Within 4 weeks of using this my junk emails dropped by 90%
Personally, I have a domain name for which all email aliases (the bit before the @) forward to my account. Whenever I fill in any form, be it electronically or otherwise, I give them a unique tracable address (eg. my address to this site is '[email protected]') - this means not only can I block out individual addresses without affecting any other email flow, but I know precisely who is spamming me, or released my address for it to be spammed.
Try mail washer from http://www.mailwasher.net/
(you will have to manually put it in cause I dont know why it didn't become a shortcut to the website). It is extremely simple to use and there is extensive help on the website.
Geoff