I’ve had my Yahoo Mail account for a long time and I get a lot of spam. Here is one way to deal with it.
Just under your “Inbox” title there is a “View” menu.
In the “View” menu select “Messages from Unknown Senders”. Now most of the messages will be spam. But go through and see if someone new has sent a message and select them. Now click the “Add to Address Book” to add known mailers.
Now when you return to the unknown senders screen you should be left with a screen full of spam. Click on the header checkbox under the “Delete” button to select all the mails then click the “Spam” button to make them go away.
You might need to do this on Multiple pages if you have a lot of spam.