World Guard (I think) is a plugin that allows player from griefing things. I don't think there is any possible way to protect it from signs. You can use the plugin Lockette to lock your chest.
If you want to keep player from breaking things,
Use your wooden axe and make the square where your structure is.
Then do, //expand vert, that will make it protect underground and above ground.
Then type, /region define spawn, if you don't want the name to be spawn, change it to whatever you like!
If you want to redo it, do, /region redefine spawn.
I hoped this help!