Hello!
Unfortunately, closing a block does not reset remembered sets of selections.
Hmmm, clearing it entirely is not possible with a trigger action, but you can certainly re-remember the set again in a separate trigger (let's name it "clear the list"), under the same name, by choosing "Create a new list every time". The only problem I see is that the trigger won't fire unless you apply it to at least one matching selection, so that selection will be in the list. But you can work around it by then checking that:
Trigger clear the list Number of runs per selection is equal to 0