Approach number 1:
Try concealing what you are doing. Don't give up your personal space. keep everything close to the vest... or chest in this case.
Pick a location that normal people( people other than artists) would find hard to get to. Be ever vigilant in the woods though, hikers like to sneak up on you.
If you are in the open try looking a little... how do I put this? deranged! People keep there distance if you look like a nut job. Squint constantly. Not only will this help you control the values in your painting but it makes people uneasy and helps to keep them at a safe distance. ( wearing weird shoes helps too)
Here's a neat trick. Set up an empty easel. Pretend you are in the process of cleaning up or just staring out at the horizon ...but actually you can be painting a small color study holding the canvas in your hand...( Artists must be creative in so many ways.)
Finally, stand in a place where they have no way to get behind you. Also dressing like a tourist and pretending to be lost while you are sketching is a good disguise.
I hope this helps any of you who are worried about getting out there and painting plein air. Of course you could just decide to enjoy the day and paint. One of the benefits of having people come over to check on you is....they might buy something.