You can block an address and Yahoo will delete any future emails arriving from that address immediately----and you'll never see any of them.
That email will NOT be returned to the sender (Yahoo deletes it), and they aren't notified that they have been blocked (for security reasons).
They can still send emails to you----but you won't be getting any of them.
So----just let them waste their time sending to you----what do you care if they know or not.
Your only other option is to tell them yourself----but IF they know, they can just change a letter or two in their address---(which allows them to slip past the block)and continue bugging you.
Mail page>>options>>mail options>>Spam (from the list on the left)>>block addresses.
Enter that exact address in the space provided>>click "add block">>then click "save changes" at the top of the page.