Fishing can sometimes be like driving. Sometimes you'll find courtesy and other times aggressiveness and outright stupidity. It's up to each of us as to how we react to each one. There have been some great responses in this thread, very well thought out and gives the reader many choices to choose from.
For me, as I near the the ripe old age of 60, and having survived this long by sticking to a life long rule of violence is never the answer or a solution to a problem. I try not to get into peeing matches with river bullies or entitled sportsmen. Life is to short and I go fishing for fun and enjoyment, not to fight.
Hell if I'm going to argue with someone I could stay home with my wife and do that.
Now thats not to say that I'm just a turn-the-cheek, pacifist kind of guy as I believe and as I've taught my kids that one should stand up for oneself. But pick your battles and be sure that the battle is worth the effort.
When I fish the Vedder or any other river I try to fish as I drive. Respectful and courteous of the other drivers around me. And if someone wants to drive like a crazy man I would just shake my head, laugh a little and continue on my way. If I am fishing and someone low holes me or moves in on my spot I would probably talk calmly to him/her and explain how I feel and test their response.
If they are insulting, aggressive or just ignore me then I would most likely continue fishing and not get into a situation. Afterall most rivers are big enough that there are lots of spots to fish and I also believe that in the end Karma will bite that person in the my friend.
At the end of the day I want to look back at my fishing experience as fun, relaxing and enjoyable and Not think about some peeing match that I could of won or lost.