So, Minecraft.

[LennyLen]

The above image was taken from within a little viewing room I set up to have a look at my mob spawner. The light you see is due to the lava kill block. I have my brightness turned up really high so you can see within the spawner more clearly. The light from the lava is really low within the spawning room itself. It has a small effect on the spawn rates on the blocks close to the lava, but not a huge amount. Mobs can still spawn there, but at a slower rate. It has no effect on spawning rates on blocks more than 5 or so blocks from where I took the screenshot.

You could always use some pistons to build a retractable shutter over the window for when you're not there to watch.