How can you lock blocks on minecraft?

Yes, actually you can! As long as you're playing multiplayer. :P just type /lwc into the chat box thing to get a list of commands, or if you just want to lock your chest without a passWord and only you can get into it, then just type /cprivate and left click the chest. :) Also works on other things! (doors, trapdoors, crafting tables, furnaces, etc...)