Those regulators work within a set range of input pressures, and need a certain delta to work properly. Over-pressure them severely and they will simply fail, and most such failures will lead to gas escaping from the regulator body. That is why you will usually find them outside of the premises they protect so they vent into the outdoors rather than into some enclosed space.

As for your bad actor: what would he pressurize your lines with? An air compressor? He'd have to dig up the lines or disconnect them first and gas from the line would likely escape in quantities large enough to discourage such tricks.

Just like in theory you could disconnect the mains from a house and then send a high voltage pulse down the feed lines, in practice pranksters and miscreants tend to avoid doing stuff that might get them killed instead.