Scarring and possibly disabling someone over it is illegal.
Bragging about it is disturbing.
I almost 100% guarantee you in court that you could easily do what Ian did and have it not deemed illegal (and get off on charges). If you gave the guy fair warning and told him he was in your casting radius (which is idiotic if you see someone fly fishing), it is their own fault. You can prove you did your due diligence by warning them, and they didn't move. It isn't your job to move, especially if you were there first.
Bragging about it however, then you would probably have some assault charges laid. I'm sure Ian though wouldn't be bragging about it on a forum if it was going to court.
I personally, walk away when some one low holes me. The Vedder is a joke, expect to get low holed, it's just the way it is out there. There honestly isn't enough water any more for people to have a run to themselves each, someone is always going to walk in somewhere. Take a note of where the fish was hooked below you, walk away, and go catch a fish there later. Low holing doesn't even bother me any more as I've caught more than my fair share behind people anyway.
Throwing rocks, really?
And yes really. If someone walks below you... fair game. If people low hole me badly enough on a river other than the vedder I do the same. Or cast my spoon on it's normal swing, hope and pray I catch their gear when they cast over me, and cut it off