Heidi Klum -- Spanks a Lot for the Ass Shot (PHOTO)

Heidi Klum's been a very, very bad girl, and the world is a better place for it.

Klum put up this pic of a bare ass getting spanked with something called "The Naughty Spoon." Impossible to say if those are Heidi's cheeks misbehaving in the shot -- but since she included the caption, "Ouch" ... we're gonna go ahead and assume it's her.

It's just hotter that way.

Care to change your vote, Trump?