That's a good question, @willmoin, and something that is especially important because anyone with edit rights to an Item Bank can delete it, per the document How do I share an item bank in New Quizzes? What I would do is select the share link for the Item Bank and see who (or what courses) have edit rights. The embedded screenshot shows an item bank that is shared with zero people, zero accounts, and two courses with one having edit rights. I would presume that the creator is someone associated with the course that has edit rights. In your example, the best option is for the instructor to select the share link and then unshare it with her course.

Share options for an Item Bank

Your question serves as the inspiration for an idea to improve New Quizzes, and that is to add an indicator on the share dialog who is the creator of the Item Bank. The process for creating ideas is described on the page How do Ideas and Themes work in the Instructure Community? 

Creating an idea does not provide an immediate solution to the problem you described, though it is a solution. I’m going to mark it as such and hope that our colleagues who have a similar challenge will find the best way to contribute. If you decide to create an idea, please link it here.