Did campsite squatters recently become en vogue or something? I can't say I've ever had or seen the issue, nor worried about the possibility.
First come first serve is no different than reserved as I see it or anything else. If you paid for the site for a week, leave and return during the day and some idiot is on it, tell them to move. If they refuse then call the ranger, host, or management and they'll make them move. I've yet to see a Fed or State campground where there was not some method used to mark a site as occupied or reserved, such as a tag on a post.
