The reason they don't "charge by the day" is because these people are NOT GOING TO PAY IT. They don't intend to join the club, nor pay a fee to fly. They think they can use the land as they please, and they are challenging you to do something about it. The first time you should remind them the land is posted, and that the club maintains the property, and it costs the club money to do so. ASk them if they are interested in joining the club and being able to fly there anytime they want. The second time they are caught there, you call the authorities and have them removed. The third time you press charges for trespassing. Sooner or later the word will get around that you are not playing games. $50 a year is not unreasonable to have access to a flying site. They will spend more than that on a lipo battery or two gallons of fuel. This is an issue as old as the hobby. It is about some jerks who want to play but do not want to pay their own way.
You are not "saving the hobby one pilot at a time" by allowing these guys to ride on the backs of others and take advantage of you.