Anti-siphon spring???

AND NOW YET ANOTHER ANNOYING STORY from the old days. Back in the 80's I worked for a large auto parts store here. We had had some gas thefts in town, and a frail sweet old lady came in. She wanted a locking cap, and we were selling those springs as well--had a display on the counter, and she grabbed one. She asked me "if there was anything else" she could do to prevent fuel theft.

Without thinking (as usual) I opened my big mouth. "Well no matter what you do, I guess they can crawl under there and drive a nail into the tank"

She looked depressed, paid the bill and thanked me and left.