Note that he says if you tell them you're not playing, they respond "It sure looks like you're playing, you're not punching me back." Which I think (at least, unless you can call in Reasonable Authority Figure and have them be punished appropriately without becoming a pariah, but actually even then) makes it a moral obligation to punch them. As hard as you can. Right in the face. And ideally then walk away, silently.

My intuition is that that's neither the moral obligation, nor an effective way to resolve the issue - I would guess that the right thing to do would be to punch back, but roughly in kind (with regard to power and location).

In the case of a "gentle tap", as Duncan describes it, I guess I would probably tend towards verbal rebuke, escalating to a mild punch if they gave me shit over it? Can't say for certain, and context matters, but it feels consistent with my behavior as an adolescent in these sorts of situations. He holds that this can get you ostracized hard, but the world's a big pond, and it doesn't really match my life experience, so I'm tentatively not buying that claim.

Escalating what is meant as relatively playful or mild violence to actual violence is probably not moral under most common belief systems, and is likely to make enemies where you could have asserted yourself at less cost.