The following instructions are for Eudora 5/6/7. Eudora 3 looks slightly different (see below).
Start by clicking on Tools and Filters. The dialogue box shown below will appear:
Click on New as shown above. In the Match section select Any Header form the drop down list and enter ******** in the empty text field as shown below. Note: you can adjust the sensitivity of the spam filter by adding or removing stars. For instance: Using **** *will increase the number of messages that will be moved directly to your Trash folder. If you enter more stars, e.g. *********** *less spam will be removed from your inbox.
In the Action section of the dialog select Transfer To. Then click on the button next to it and select Trash.
Finally close the Filters window, collect your E-mail and check that mail labelled as Spam goes directly to the Trash folder.
Eudora 3 has a filter option with the same principle, but it looks like this: