How To Beat the Siphoners

I live out of town aways. About 1000' in front of us, down the hill is Lake Spokane. Neighbor has said he has had gas go missing once. Following weekend had a knock on his door, it was the siphoner coming to replace the gas he took. His boat ran out of fuel the week before and he "borrowed" some from my neighbor, who was at work....nice to know not everybody is a jackass.......