I thought of "no trespassing sign",But it may trigger my immediate neighbors ,all of my neighbors are nice.It may indicate the sign is for one of them.These people that walk on my lawn are not my neighbors. I am hesitating to put up that chicken wire fence you suggested, I worry what my back door neighbor will think.They may assume I have a beef with them and being passive aggressive .I have already hinted by putting logs across where they were crossing my property. It didn't work and they are simply jumping over my logs.The older boy simply lifted up his bicycle over the logs and continued bicycling across.