To make all the scenery in a map disappear in Forge mode, go to a map that has a lot of scenery (for example, Foundry) and make a trip mine. Set the Run Time Minimum and the Run Time Maximum as the same number by looking at it and pressing X while you are the Monitor. This will make it have an instant respawn. Stand on the trip mine and place it as many times as possible. After awhile, everything that can respawn will disappear. Everything will come back eventually, but have fun with a big empty room. This works best if you get multiple people and make multiple trip mines. You can also use this trick to create extremely massive piles of trip mines.