That's a good idea, I thought up something sort of similar to this.
How it would works is, for people who do not have your trust have a default time limit set to their bricks, that number being 10-minutes. Vehicle spawns are 10 minutes, weapon spawns, etc.
After the time expires the server auto-clears the untrustee's lonesome spawns.
You would have a GUI that allows you to set the number of bricks that is needed to nullify the auto-clear (forcing whomever to actually build something without it going unnoticed by you) and the default time can be changed.
A counter could possibly hover over the players name once the brick has been laid so that they can just tab out and see how much time they have, if they need more they can request it and you can change the time you have alloted them.