As far as I can tell from Eudora's website it should be possible - you can filter on the content of any headers, so you just need to look at the emails you are getting and see what the header lines are that are common to the mails you want to reject (for example they might all contan the characters "BIG5"). You can basically write any regular expression (logic statement) to filter messages and decide what to do with them.
I use Turnpike myself, which is quite similar.
The ads in the free Eudora might be a bit of a pain though.