You can't. Yahoo lets spammers have email addresses - this was deliberate policy from the top for a while, and has often been done since then by their dishonest employees or their totally incompetent security. Once the spammers have got the address they can fake mail from it indefinitely.

The nearest thing to a fix is to send out a mass email saying that the Yahoo address is dead and that every potential recipient should treat anything purporting to come from it as malicious.