Eudora 5.0:

Click on Tools

Select Filters



Click on the New button in the lower left hand corner



Select the Incoming check box at the top of this box

Under the Header section select Subject

Below that select "contains" and in the box to the right of that type *****SPAM*****

Under the Action section select Transfer To and to the right of that select Trash



Then click the "X" in the top right hand corner of the Filters box and it will prompt you to save changes to the filters, Click Yes on this box.