Signs. They are the first step and are needed. Some that say "No Flying except by Members and their Guests" ; another that state the Clubs name and the requirements to join, safety requirements and room for announcements. And others still for General Public so they do not wonder on the field. I know signs make a good target so make replaceable ones. Signs and safety fencing makes the field look much better in my opinion, it looks like a model flying field instead of a hay (etc.) field.

At 50 dollars to join it sounds like a good deal. Some people need to be asked to join a couple of times, explain the benefits to them and you. Have a "fly-in" to show your club off, and invite all AMA members to fly that day; at the fly in be setup to take instant members.

Are these repeat individuals? Are they young (don't know any better yet) or old (to set in their ways)? Are they damaging the place? Did they fly there last year? Are they some sort of local official, or the owner of the property or adjacent property?

As for the other comments about use of force, it is not a domicile nor a farm/ranch; besides it is not good to terminate potential new members. Even calling the local authorities has a real downside that should not be over looked; but sometimes calling is needed. You want to be inviting not necessarily to these scofflaws, but to others including your current members and their families; actions you take against some individuals effect how others view you (even unfairly).

If they are real pains take pictures and post the pictures of them on a sign and say this means you.