I'm not sure that you are contacting the correct entity. Dreamhost doesn't have any control of what content its customers choose to put on their sites. You say is defamatory, but does it specifically violate Dreamhost's Terms of Service?
You might also want to note paragraph 5 of Dreamhosts Terms of Service:
While you see the situation one way, someone else see's it another. Dreamhost can't take sides, or decide which side is "correct". Dreamhost will always act first in it's own best interest, second in its customers' best interest, and lastly everyone elses. If it doesn't violate Dreamhost's Terms of Service, then you will need to get a court to order for dreamhost to act.