Which destructible did you base those objects on?
The fact that you can't remove them is most likely because destructibles do not disappear once they die: they are "soft-dead", so to speak.
Removing the destructible once it dies is a good idea, but I doubt there's an event for that. You can only have it trigger with a single object, which means, the only event that detects the death of a destructible asks for a specific object, which you have to select while creating that event, in the trigger editor (I hope this was clear).