When I've run events before, I've tried to set out signs with plenty of warning for any locals coming through. I'd also post the dates on any online resources I knew, but I didn't really expect any casuals to be checking that. At one of the courses I used to host at, it was actually in the parks department's stipulations that the course could NOT be reserved, which made things a little tricky. In my "warning signs" that I set up beforehand, I kind of implied the course was reserved, but never explicitly stated it. It seemed to work.
Another time, I was helping out with an event at another course, which at the time, was one of the bigger ones in the area. The TD hadn't bothered to set out any kind of signage that there was a tournament in progress because naturally
everyone knew about it. Just before the first round was about to start, I jogged out to the farthest tees to make sure everyone was in place, and saw a group of casuals playing up the middle of a fairway. Great. I didn't want to be the bad guy, but since I was the assistant TD, the onus was on me.
I walked up to the guys and told them that unfortunately, they'd have to find somewhere else to play because a tournament was about to start. They asked if they could just play between people, and I told them it wasn't really an option because the course was completely full, with 5 people on each tee. They started to get a little hostile then, saying stuff like "well, you can't stop us anyway. It's not like you can close the course down." To which I told them that actually, we could--the players in the tournament had paid money out of their entrance fees in order to reserve the course (which was true in this case). I told them that they could try to play during the lunch break if they wanted, but the course would be slammed most of the day. The guys weren't too happy about it, but moved on. They'd driven an hour and a half to play, and were rightfully upset.
Sure, they could have found out that a tournament was happening if they looked online somewhere, but these guys probably had no idea that there was even a place TO look. I mentioned it to the TD after the tourney got under way, and he was like "oh good point, maybe I should do something next time." It hadn't even occurred to him before that that people might be oblivious. A lot of times, "serious" discers and even moreso DGCRers get caught up in the bubble of the scene, and don't realize how little the rest of the DG population knows or cares about tournament golf.