Never had any real sure fire way of keeping people from taking over my site when I was gone. Heck they'll even take your "Site Occupied" sign. Once we had gone into town after registering for a site and within an hour and a half, came back to find someone parked in our site. Told the manager who said they "accidentally" placed another party there. So, they told me I could park in an area behind some bushes and some buildings instead of the ocean view site we were originally promised. I told him where he could "park" it!