As for the signs causing more people to mess around in the field, I have to wonder. You say this happened two weeks ago, what has happened since? I believe we will continue to have isolated cases like this and the only way to try and stop it is to be pro-active, meaning call the sheriff as soon as you see trouble or something that affects your peace of mind. After all, you bought your lovely property that is backed up to that county property with the intention that you won't be bothered by anyone behind you. Or, at least you began to think this soon after moving in. I feel it's a nice little extra value for our property and it's up to us to monitor it.
If you would keep my number handy in your phone and would like to give me a call anytime something like this happens again, don't hesitate to call me at 813-961-5010, day or night. If I'm awake I have my ringer on, otherwise I usually turn it off but then leave a message and I'll get back with you to discuss what happened. Don't forget you can call to report anything to the sheriff as an anonomous person, you don't have to give your name or address although I've heard it helps to give them this information as they have to come out and meet you. This also is in my opinion a starting point for the sheriff to come and get specific information on what you're reporting, location, time, identifiying info, etc.