You can delete your account by going to your profile page and clicking the settings tab. At the bottom of the page you'll find the Delete my account button.
Note that you can only delete your account if you don't have any pending or confirmed bookings. If this is the case, you could cancel them before deleting your account.
Similarly, if you are an instructor, you can only delete your account if you don't have any pending or confirmed bookings for your skills. If this is the case, you could decline / cancel them before deleting your account.
If you want to learn how to unregister as an instructor, please read this article.