Click "Edit Topics"
Note that if students have already begun the activity that you're editing, changing the number of topics may affect existing scores, and students will need to revisit the activity for their grades to update. Be sure you want students to do this before proceeding.
Here you can hide topics that you don't want students to get questions on. Removing (or adding back) topics automatically adjusts the target score by 150 points.
Click "Save changes" when you're done.
Note that you'll need to keep at least one topic.
Also note that quizzes with only one topic tend to be small, and your students may experience repeating questions.