with a lock
It's very popular:www.instructables.com/id/Lego-Combination-Safe/ L
www.youtube.com/watch Here are instructions for a safe thats simple but good!
You don't. You go buy a real safe since you can take apart lego safes.
Aren't they already safe? I mean, they wouldn't sell them to 5 years old and up if they weren't, right? As for locking, well, thats how they connect and stay together.
Take away its weapons.
with lego bricks?google FTW!