There is no way to do this on the iPad. All you can do is mark each spam email as junk. This will 'eventually' take care of most of it.
However, depending on your email provider you can set up rules on your account that will do this. The nice thing about rules on the server side is that they take effect before you're device downloads the mails.
To do this with an iCloud account you need to visit icloud.com from a computer. Google is similar. I don't remembe if Yahoo has rules, and I have not experience with other email providers.
From your description it sounds like you are using something else. It would be strange if a service that had the ability you mentioned didn't also have the ability to move email to a specified folder base on a rule (like SPAM) being in the subject line.
Good luck,