Never heard of the purple paint either. That may not hold up in court, or at least in all states.
We did put up signs high with ladders. They still got tore down. If you are hanging them on trees, it usually means fallen braches are lying around. Easy to reach up and smack them down. If that didn't work for them, they just shot the h-## out them!!!! Just be persistent....
If you get someone to watch over the place, I'd bring in an outside friend and give them privileges. I sure wouldn't approach someone you caught trespassing to help. They are unscrupulous already, do you think they can really be trusted when not watched carefully.
Having been thru multiple avenues in dealing with ridding trespassers and the grief and fear this gives, I now think that is better to stay firm/hardline and it ends quicker, in my experience. When we did the softer approach, it lasted 60+ years and never worked.
