To achieve this you could add a boolean to the object you're looking to randomly pick from, something like "Picked_Before" - which is then set to true when the object is picked. To pick a not-previously-picked instance, put the condition Not Picked_Before directly above/before the pick random instance. If you do something unique to the instance that has been picked (put it on a different layer, set a different animation or behaviour etc) then you could use that as the first condition filter and there would be no need for the boolean.