Minecraft wiki entry for Spawner, associated text: A spawner is also disabled by filling the spawning volume with solid blocks. This is often used to disable magma cube spawners, as magma cubes can spawn in any light level. However, since all magma cubes require the space of a large magma cube in order to spawn, a clever placement of as little as 9 solid blocks a layer above the spawner (see right image) can completely disable it.