I missed that you said "forwards email to Outlook 2013". What I described will (obviously) not stop forwarded emails.

Unless there is some way to filter what gets forwarded (on the Yahoo side) I don't know how you would stop the SPAM from forwarding.

The only other thing I can think of is a Rule in your Outlook that looks in the Message Header and (somehow?) determines that it came from the Yahoo SPAM folder, and then Delete it in Outlook.

See "Filtering Based on Customized Headers" in
https://its.uiowa.edu/services/micro...m-heading-1511