can't luff someone- ie force them head to wind at any time during the race.... only prior to the start. you can't do it AFTER the start. "luffing" doesn't apply to the case timbo is talking about.
I need to look it up too, but I think you can force the windward boat to the corner of the line and force them to slow and duck you. The mark is considered as surrounded by navigable water so it is not an obstruction. I would have done what timbo did- let him in (because I am not into pissing people off like some others).