better idea: build floors out of gravel or sand, suspended underneath by torches, placed over vast pits filled with cacti. then build normal stone/wood/etc. ceilings, and 1 block above that, place stacks of gravel/sand.
if they dig down, they fall to their death. if they dig up, they are squashed.
for extra protection (and I actually DO do this sometimes, for fun), build all your walls 3 squares wide. The outer blocks can be pretty much anything non-flammable. The center wall blocks are lava. Someone fucking with your wall? Not for long.
(Or just line every single interior non-flammable floor, ceiling, and wall with lava. Surprise!)
There's a bukkit plugin that lets you quickly make doors out of blocks, manipulated by switches or levers. You can have a looootttttt of fun with those.
I also made a set of griefer prisons out of bedrock, lined with lava :x. I have a command that can teleport someone to a named location. So if someone pisses me off ... Heh