PC based email programmes allow you to block the senders of unwanted emails, sending them straight to your Junk file. How to do this depends on the programme you use (Outlook, Live Mail etc.) Try Googling "(your programme) block senders" and instructions should come up.
For Windows Live Mail (mine) it's easy:
Highlight the message in the list.
Make sure you are in the 'Home' ribbon (top of the screen).
Open the 'Junk' drop-down menu and select 'Add sender to blocked sender list'.
All that sender's messages will now go straight to your Junk file.
You can also block the sender's domain and this is very effective against domains registered specially for junk mail, but do be careful here. If it's a general domain it could block wanted emails from others.
If I remember right, the 'Blocked Senders List' in Outlook Express can be found under Tools - Message Rules. This might help: Block Senders in Outlook Express
Or try this one covering Windows programmes:
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