If you use Outlook Express go to Tools/Message Rules/Mail and create a rule to block the sender or, highlight a message from the sender you want to block, then go to Message/Block Sender.
You can also block spam directly at your ISP mailbox.
Go the your ISP.com address,and click on mail;you will probably have to enter you email address and password .
When you have done this you can make ANY incoming email into spam at this mailbox,and it will never reach your Outlook Express any more.