There is no way in the iPad's native Mail app to mark junk mail as spam; this needs to be done on your email service provider's mail server as a 'rule' there.
To solve this issue I opened my AT&T U-Verse home page went to my AT&T Yahoo mail then added it to our iPads Home Screen. From there I moved it the bottom APP row. Once on the iPad the AT&T Yahoo Mail takes on the look of an iPad app. Love it!