Ahn'Morpork: Placed to one side
Bloody Stupid Johnson works like this: 
You roll for a random city area. When you roll an area that contains a building, the owner of that building has to put the corresponding area card away. The card says "to one side" but the important thing here is that the area card is removed from play. The building is still there, it still counts as a building for all intents and purposes (win conditions, for instance), but the owner can no longer use the power on the area card.

According to Martin Wallace, the designer of the game, the card is back in the game if the bulding gets destroyed and subsequently rebuilt (by the same player or another).