Temporarily disabling a mob spawner farm?

Solution 1:

If you want to do it completely safely and the roof of the spawning chamber is accessible, you could drop gravel or sand through the roof of the chamber to obstruct the spawning area on top of the spawner. If you want to redstonify it as well as fix the spawning issue, I'd suggest swapping out some roof tiles for redstone lamps in the chamber, then place your block of choice on top of the spawner. I usually place 4 lamps in the corners of a 5x5 grid on the roof, centered on the spawner, like so:

L O O O L
O O O O O
O O S O O    O - Any Block
O O O O O    S - Spawner is under this block
L O O O L    L - Lamp

Additionally, I usually put some redstone lamps in the walls as well, but provided that the light from the ceiling lamps are enough, they are unnecessary.