Get a command block and place it somewhere so the players won't able to see it (like behind a wall). After that type in /gamemode adventure @a. I would recommend doing this if you are already finished with the whole map, If you put it in the process of you building it, then you will keep switching to adventure mode. If you accidentally typed in the command inside the command block, then you could turn off "command blocks"

Just set all Players into Adventure Mode: /gamemode 2 @a
Then you're not able to place or destroy Blocks