In order to delete a response from a class, visit the activity feed for that group and search for it by scrolling or using the search box.
When you find it, use the "X" button below the right corner of the image or the title, circled in orange in the image below.
Clicking the "X" will prompt a confirmation. Click "Yes" to permanently delete the response.
If the response was created in multiple groups, you will need to visit the feed of each group you shared it with and delete it separately.