If you leave the valve attached to the mains line then you can access the house lines, usually there is even a special port for this that you could use without disconnecting the mains line that is used for leak inspection (they evacuate the lines and measure the rate of seepage). Once you connect something in the line and you open the valve again all pilot lights will have been extinguished and won't re-light due to safeties.

Anyway, if you want to destroy someone's house there are much quicker, less obvious and easier ways to do so.