how to prevent clicking gameobjects behind NGUI panel

we have like a city building game we are working on, we are facing this issue where, when we click on the ui in the game-play screen and if the ui is above some building, it's also clicking the building and opening the building's menu, all this is done in 2d using isometric art (2.5D), does anyone happen to know of any solution? We are using NGUI and we are not using unity ui.