You can now deactivate questions from your all of your students' personal decks:

By default, all questions are active in a personal deck.

To go back and deactivate older questions from the personal deck, go to the "Edit Questions" area of an assignment, and then navigate to the "Existing" questions tab. Each card has an indicator of whether it is "active" or "inactive". Once you open the question, you can then deactivate that question.