The frontend has no way of knowing what is or isnt installed since they run off a romlist have to either delete it via editing the romlist on your regular pc, or a keycombo of buttons to bring up an edit menu to "delete" a game.
I was going to make it automated with another preloader to check files and folder, and externally edit the romlist, but i went against this due to the folder system being different in nikos multi , so i opted with an error message that gets displayed when a game cannot be located, and it states what it was trying to load.