Spam is tricky. Especially the recent waves of spam, because while they all have a similar source and there is similar subject matter, the sender will almost always be a different email address.
You will most likely have to deal with the influx until the Yahoo mail servers can update and improve their filtration methods to cover the new sources (there may be an open SMTP server somewhere that they are all generated from and the mail servers can block IP blocks or whole mail servers).