You probably cannot stop it, but maybe you can manage it.  There are programs that blocks email unless the sender is on your approved list. One example is Choicemail ( http://www.digiportal.com/ )

edit:  Oh, and as much as I like Outook (I use it a lot on my non-internet computer) I'd strongly suggest switching to Thunderbird ( http://www.mozillamessaging.com/en-US/thunderbird/ ) or something else. There are just too many crazies taking potshots at Microsoft products to risk it.

Another possibility is filtering.  I have one person who just loves to forward junk.  I am using gmail, so I went to the gmail site and set up a filter so that anything from his address goes directly to trash.  Once a week or so I sift through it and read anything that seems worthwhile.  Lavabit seems to have some features like that you may be able to take advantage of.  See http://lavabit.com/features.html

Server Side Sorting

The Lavabit e-mail server allows you to automatically sort your e-mail using a powerful regular expression engine. Imagine having your e-mail pre-sorted on the server into specific folders. For those of you who access your e-mail from multiple computers, this technology means you’ll now see a consistent view of your e-mail.