I will not use fertilizers, insecticides or weed repellents that will in any way affect other plots. I will not bring pets to the garden. I understand that neither the garden group nor owners of the land are responsible for my actions. However, the fear tends to be much greater than the actual incidence. Let people know to whom the garden belongs and that it is a neighborhood project.
Fences can be of almost any material.
They serve as much to mark possession of a property as to prevent entry, since nothing short of razor-wire and landmines will keep a determined vandal from getting in. Short picket fences or turkeywire will keep out dogs and honest people. Create a shady meeting area in the garden and spend time there. Invite everyone in the neighborhood to participate from the very beginning. Persons excluded from the garden are potential vandals.