It is all too common a problem. I've experienced this where a guy and, presumably his father or g'pa in the back. This was on a nearly 300 acre reservoir. Buddy and I were fishing the docks and this dude, who arrived after us, decides he wants to as well. So he motors inbetween us, within 15 yards of me, and about 8 of my buddy.
I caught a lovely 3lber right in front of them, then moved to the other side. They decided to follow me. The young doofus thinks he owns the spot even though I was there for nearly 10 minutes already, and casts within feet of my yak. I could almost feel the splash of his bait. I kept giving them the stare of death hoping it would work, but the dummy returned stare. Finally I looked at them and said "ever heard of fishing etiquette?!" The kid mumbled something, I said I was there first, blah blah, and I paddled off.
Idiots are out there. Since then my buddy has experienced very similar rudeness from other guys. BUT!! There are PLENTY of decent fishermen out there, but it is frustrating to encounter the few jerks.
Some guys on another forum suggested I carry a nice 2oz weight and cast it over at them... LOL. But then, like Sam said, you never know what kind of people you're dealing with.
And my wife is a nurse who deals with psych patients sometimes. She told me of research that suggests we'd be very surprised how many people in "normal" life have psychoses, i.e., significant mental imbalances, not just the "I'm on antidepressants" kind. A lot of the people who come in to the hospital ER for quick mental "fixes" over breakdowns, etc., are everyday people you'd see at the mall, on the road, etc., but have these mental issues. They may not be taking their drugs, or such, and have "episodes".
Point of it all is to confirm/add to Sam's comment that you never know what kind of person/people you're dealing with. Are they just rude, or have other issues which would make them snap if you confronted them.
My wife is urging me to take the "shut up and move on" approach. And even though I'd rather throw a few choice words back at the dudes, I am seeing the wisdom of her counsel more and more to take this stance.