That etiquette goes both ways. There are a few 18 hole courses here in So Cal that are pretty much just that, DG course, yet, people still wander in or around them.
In anaheim, you mostly find those "foreign" folks playing some futbol soccer in some of the fairways. If we can't get around them, we just skip it, and sometimes use "make up" holes. We just make them up where we are clear from laypersons and move on. It sucks, it's sort of "safari disc", but it works. What I do run into quite a bit in Anaheim (Reid Park - 9 hole) and Tustin (frontier park - 9 hole) is that the "foreing" folks tend to pick up the disc you just threw in their direction and they try giving it back to you. That is when i very patiently explain to them what I am playing, how it works, and move on.
I was playing at Irvine (Deerfield - 9 hole) one time and there was another pair out there playing a round as well. These guys were the type that extend every tee back about 50' - 100' from the T-pad. I guess Deerfield was previously an 18 hole course, but from what I understand, was too crowded and a few laymen got hit, so they are trying out this new 9 hole. So these guys are playing what I would assume would have been the original tees. Anyways, there was a guy teeing off from the "official" tee and these guys were like yelling at him to get out of the way, almost offended that this guy got in their way.... I saw him yelling at a pedestrian later because that person was in their way. Some people are just playing A-Holes!
I really do try to educate those that are not aware, and keeping it cool works out, or has for me. Of course, I have only been playing again for about a year, so I know some of you have logged more course hours....
Some people are just unaware of the course. That's all. Education my friends... it works... sometimes!