Deleting a exercise is done within a few steps, as shown below:
The steps to take are:
Log in on the platform.
Make sure you are on your course map (click on Courses in the right top corner to be sure).
Click on the repositories button in the upper left corner of the screen.
Click on the Exercises tab to view all your exercises.
Select the exercise you want to delete.
Click on more at the right upper corner and click on Delete Exercise.
This will permanently delete the exercise and make it unavailable to you, the people you shared it with and your students.