03/31/14: Some Pinners in safe mode are having trouble deleting the spammy Pins that appear on their board covers. Sorry for the confusion! We took the Pins down on our end, so they're not on your boards anymore.
The Pins aren't being removed right away, so the best way to fix this in the meantime is to Pin 4-5 Pins to the specific boards. Then, the small thumbnails of the spammy Pins shoudl go away.
If you haven't already, make sure you reset your password to help secure your account.
Here's some tips to help keep your account secure: https://help.pinterest.com/entries/21686711