Once you append some string to the summary text, it's difficult to remove it. So I suggest storing all parts of the summary text in different variables. And then assemble the summary text from those variables.
Also, all your events are running on every tick, this is not a great way to do things in Construct. You need to use "ListBox On Selection Changed" event, and compare selected item in sub-events. And remove all "trigger once"
AppleOrOrangeList On Selection Changed
...AppleOrOrangeList selected item=0 : Set fruitVar to "+2 Apples"
...AppleOrOrangeList selected item=1 : Set fruitVar to "+2 Oranges"