Minecraft wiki entry for Hiding_chests, associated text: One way to trick players into not getting your loot is to have a room that seems like it would have all your valuables in it, but instead only has junk. Hide your loot somewhere else entirely, where players would never dream that you would hide it.
When using redstone in hiding chests to make a deadly trap, lava is a great option. Lava does not destroy chests, but at many times it is deadly to the player. However, make sure your chest room isn't made of wood, or the whole room will burn down!
If theft (and/or possibly griefing) is so commonplace that all above solutions fail to reliably protect your valuables (either due to server rules or poor administration) regardless of circumstances, a final solution might be to spread out your valuable items across several different locations instead of storing everything in one place. This allows you to minimize property losses incurred by theft or looting as if any one of the stashes is raided, it should hopefully contain so few valuables that the resulting losses will be manageable.
If you trick the thief with the first trick, you will find out that the griefers are suspicious, and in some cases, may try to find the real storage. You should put 1 diamond and some iron ingots in the junk chest, so the griefers won't be suspicious.