This photo was taken near the entrance to a village somewhere in the New Territories. This is the first time I have seen such a notice, but its meaning is obvious enough. 「伯公」is the name of the the spirit that supposedly guards the land. Alternately he is also called 「土地公」,「福德爺」,「福德正神」, 「大伯爺」, or「后土」if he is supposedly guarding a graveyard.
As to why worshiping by outsiders is forbidden here? I don’t really know. Perhaps the villagers are afraid the “benefits” may be usurped?
This idea of exclusive worship is quite alien to us Christians. We believe God loves all mankind. We know God by His grace which is freely given to us. Hence we would love to share it freely with all those who are willing.