MInecraft: Spelunking for beginners #1 "How to suppress a mob spawner economically"

The new terrain in 1.9 is riddled with caves, ravines, abandoned mines etc and I've noticed I'm finding a lot more dungeons also.
After watching many LPr's I cringe when seeing how some (naming no names) tackle mob spawners. So many people watch these guys they may be picking up bad habits so here's my first spelunking tip on how to suppress a mob spawner economically.