Try this. Instead of selecting the email to read, (while in landscape) tap on the edit button at the top right of the email list. Now select the offending email and tap the delete button.
Another possibility, go into Settings >> Mail, Contacts, Calendars, and open the account. Now turn off the Mail using the switch next to it. Go back to the Mail app and make sure your Yahoo account is gone. Now go back into settings and turn Mail back on.
This should force a complete re-sync of all you Yahoo Mail folders on the iPad. If you've deleted the spam email online, then it won't be there to re-sync.