Quiz

Another suggestion is to edit the "bad" question right in the quiz. Moodle does not stop you from editing the questions while the quiz is open, it just won't let you change the quiz. In addition to changing the scoring, as Tim suggests, you can change the text to something like "Please move on to the next question, this one does not count", and change the responses to generic placeholders. That would make sure that students don't get bogged down trying to answer the bad question before moving on to the rest of the quiz. You might even change the name of the question to something like "DO NOT USE" or change the question category similarly, so that you don't pick it next time by accident.

One caveat is that this will change the question in the question bank. Therefore, if you want to keep the question (make it better for future quizzes), you might want to edit twice, using Moodle's capability to save your edits as a new question before making the above changes.

If the quiz is still open, you can also "fix" the question, but then you have to worry about how to deal with attempts that have already been made or are in progress. Whether this is a good option depends on how many attempts have been made and what the goals of the quiz and/or class are.