There is no way to stop 100% of the spam from getting into your inbox and even trying to filter any spam creates the risk of filtering messages that were not spam. However some software or carefully chosen words to filter on can greatly reduce the amount of spam you have to manually delete without great risk of accidental deletion of real mail.

Outlook allows you to create rules through the "Rule Wizard" under the "Tools" menu item. With these rules you can set outlook up to automatically delete messages with specific words in the body, subject or address names. Examine the spam that you receive and find words that are unique to the spammers that you could not imagine being in a legitimate e-mail.

There are many third party products that have complicated pre-configured and customizable word lists. For a corporation the product I recommend is from GFI called MailEssentials which boasts a 98% accuracy in detecting spam.

Once your email is targeted for spam there is no way to stop it. Unsubscribing is very ineffective and may actually increase the amount of spam you get by validating that the e-mail address works.

Another way to minimize spam is to not use your normal e-mail address when you fill out forms or post it to message forums on the internet. The world wide web is harvested by "robots" for e-mail addresses to be added to spammers address books. You'll likely find any e-mail address on your website receives much more spam than those which aren't. I suggest setting up special e-mail addresses which you realize are going to be targeted for spam and use those for forms and forums.