How can I prevent mobs from spawning in unlit places?

In vanilla minecraft mobs don't spawn if the light level of the block is 9 or above. However, I often don't want to fully lit all my structures. I might want a courtyard or a cellar that isn't fully lit. Also controllable light systems with redstone are a lot of fun to make, but I can never turn the lights off in fear of spawning mobs.

Is there a mod or another solution to have both unlit places and no mobs around? Maybe some kind of block that prevents mobs from spawning within a radius.

Solution 1:

Not counting mods, the best way I know of is to make sure the area you're "protecting" has a floor made of blocks which don't allow mob spawning.

Any transparent block will do the trick, such as glass or snow, and in your case maybe Slabs will be most suitable.

For more in-depth information about spawning and spawn conditions, check the wiki: http://www.minecraftwiki.net/wiki/Spawn#Mob_Spawning