Click on the Tools menu, then message rules. Click on the Blocked Senders Tab. You should see a screen similar to this one.

Click on the Add button. In the window below, enter the Email address of the sender you intend to block as well as the services you wish to keep from seeing.

Click OK. You should now see the information you've just entered in a window similar to this one. From here on in, any services that you have blocked, in respect to that particular sender, will not be displayed.