Not a teacher.
A "slap" is a physical blow across the face, on the cheek. With the hand open, not closed in a fist.
It is most commonly used as a firm rejection of a behavior, be it a child doing something naughty, or a man trying to take sexual advantage of a woman.
The idea here is that this "Walt" needs to be brought back down to earth. That's another use of the slap, now that I think about it. The slap to bring someone back to his senses.