block key words. I am unsure if these will show but the common ones are penis and such forth. If you use outlook just set filters to block unknown addresses containing those words.
I realised it best not to post the words I have used to filter mine but 10 seconds looking at these emails will give you a few ideas what to block. V1agra V 1 (ONE) agra is another common one to block along with the normal viagra spelling.