Hi Janice,
Sorry for the minimalistic explanaition.
When you are logged in to your account you click "Options". Then you click the link "Email rule" (under "Mail handling"). Then click "Add rule". Fill in the first part of the (or the complete) email address you don''t want to recieve mail from in the field "the phrase..." and chose "Trash" instead of inbox in the field "then deliver to...". Click "Add rule" again.
Now all emails from this sender should go directly into your Trash folder.