Why do you want to block it clientside?

IhateSpam and SpamInspector are 2 good client side utilities.

Although, We've found users take just as much time fooling with these client side programs as they would just moving the junk mail to the deleted items folder.
Or they are constantly checking to make sure the programs didn't move a good email to the junk email folder.

If junk mail is becoming a problem you should talk to your boss and have him relay it to the email admin. My best suggestion would be to change your email address.