My son has an issue with headbutting when he gets tired. Sometimes he gets me really good when we are just goofing around before bed. I try and startle him a bit when I know he did it on purpose to let him know that it hurt, even if he may have missed. Just by saying "ouch" loud enough to surprise him the very second he does it. He's only just turning two and it's starting to abate now that he realizes he can hurt people with that action. Just the action of letting him know it hurts and explaining to him why he can't do that is enough. No punishment is required because, like you say, he doesn't realize.
The same with spilling things. He used to spill cups or bowls just because he was curious. Now, after simply showing him that it's not appropriate and someone else has to clean up his mess, he realizes even when something spills on accident now and tries to clean up himself. There's no need for physical punishment, just guidance and patience as they figure things out on their own.