How do you stop a Pharoah's Serpent reaction?
Just put water on it.  A temperature of $\pu{165^\circ C}$ is required to sustain the decomposition. Adding water will remove heat from the reaction and will dissolve a small amount of the mercury thiocyanate. The heat removed by heating and boiling the water will cool the contents below the decomposition temperature.  Adding soap will help the water penetrate into the powder and more quickly stop the reaction.