<<Doesn't ignoring it teach the child they can get away with it?>>
I think if you ignore the behavior, it will teach the child that they won't get what they want from the behavior.
And to the PP who asked if the parents hit so that's why the kid hits -- you obviously do not have experience with a kid who has such severe disabilities that they engage in destructive behavior. My son has scares on his arm from his biting himself when he gets frustrated. I can assure you he has never seen his parents bite themselves when they are frustrated, he came up with this one on his own, unfortunately.