Minecraft wiki entry for Nether_survival, associated text: While a blaze spawner can be destroyed as usual with a pickaxe, you probably shouldn't do that unless you're sure you'll never, ever, want to hunt there. Blazes can spawn up to light level 11, up to 4 blocks away from the spawner (and a block above or below it), so simply placing torches on the spawner doesn't stop them from spawning. You'll need to plaster the area with a mix of jack-o-lanterns (or glowstone, shroomlight).