How to stop mouse input from be detected through GUIs?

I am trying to make a simple GUI for my game, but I keep running into one problem: Mouse input is being detected through my GUIs! What I mean is, if I make a popup on the screen and it overlaps other guiobjects (such as buttons) those buttons can still be selected and clicked WITH the popup overlapping them! I know this for a fact because I tested it and it also showed my mouse as black when I hovered over some areas of the popup, meaning that it is detecting the buttons under the popup. Does someone know a property I could set or a script I could write or a GUI I could make that would stop this? It is very annoying and I can't move forward with my game until it has been fixed.