You could always set the ComboBox.Enabled = false 
But something tells me that's not what you want to do. There's not a way to prevent it from happening, but when it happens, you can immidiately hide it by handling the DropDown event of the combo. If the condition is false, then immidiately set the DroppedDown property to False. There isn't a Close() method or I would recommend it, but the DroppedDown seems to do the same thing.