Mark English

Chosen answer: As Spongebob explained in the certain episodes he has no bones so the force of the flats punches have no effect. To be more technical there is nothing solid that Flats can punch into and hence why it has no effect. Spongebob has never really been injured and there really isn't any incidents I can recall that his "spongeness" ever came into question of being hurt or injured.

Lummie

Answer: He does have bones in some episodes.

Dan23

Those episodes and scenes were probably just jokes.