Build an Accessible Drag and Drop Interaction in Storyline

I see you changed the state of the top button to hidden when the option has been used, which removes it from the tab order. You could do the same for the definition, which would remove it from the tab order as well, preventing a keyboard user from selecting it a second time. You would need to add the definition object on each of your extra layers as well so that it is visible and readable, but then it would be a non-interactive object and not pickable again. Note that you would also need to remove any extraneous triggers for that visible object, because if there are triggers it is considered interactive and will be tabbed to.