There's a server pref called $Pref::Server::BrickPublicDomainTimeout. It is the number of minutes it takes for bricks to become public domain after their owner and all of the people they trust have left the server. This way dedicated servers can be at least somewhat self-cleaning without admin intervention. The default value is 15 minutes but it should probably be something like 2 or 3 hours. Just set it to a really high number if you don't want this to happen.
In addition, you should only full trust people that you actually fully trust. Build trust lets people build on top of your bricks but does not let them destroy your bricks and is a much better alternative. Currently, painting is considered a "build trust" action but I am considering moving it to "full trust"
