Defensive Debris will stop ambush, playing events, bringing cards out of shadows, as well as card abilities like Qotho. It will not stop cards returning to play if an effect that removed them from the game expires (for example, Coldhands).
If you choose a card with printed cost X, cards with printed cost 0 are also affected, and vice versa. If you choose a card with printed cost "-", no cards will be affected, because no cards have the same printed cost. For instance, you could choose A Pinch of Powder and still bring another copy of it out of shadows in the same phase.
If you have multiple copies of Defensive Debris in play, you can trigger each of them in the same phase. (See Limits and Maxima.)