I've had it happen many times, I usually will make some kind of noise and wave my hat. During general firearms season I've had folks walk right under me before it was even legal light, and stop right there......I have also done it to other people and when I realize they are there, I turn around and go somewhere else. Only time I have ever kept going was when I started to turn and realized that I had already walked past a guys hunting partner who was behind me. I sat there looking at both of them shrugged my shoulders and walked about another half mile before setting up my stand.....Killed a nice 3 pointer (my first antlered Buck) that day.
Many years ago I was VERY high in a poplar tree overlooking a main trail (wide enough to drive a truck down). I set up there because it was a doe day and I figured folks would be running dogs and it was a good escape route for the deer. There was a group of hunters walking along, dropping off a team member every couple hundred at and they had one stop right beneath me on the trail. I never said a word or made a noise until I opened up with my 30-06 and killed a deer that the dogs had pushed our way. By the time I got down from the tree, he had packed up his stuff and left...Guess it tightened his corn hole just a bit. Public land is public land. I try to be respectful of other folks and give them as wide a birth as possible. Sometimes you just have to change your plans.