A Superuser can delete data from an entire Instance. An Administrator can delete data for a specific Institute. A Teacher can delete data for a given exam.
Please note that deleting a candidate from an exam can not be undone.
For deleting a specific candidate from an exam, you can access the exam via the Administration panel. Select the exam and access the Students section and click on Manage.
You will select the bin icon corresponding to the candidate you wish to delete.
Alternatively, you can click on the red Delete button in the Administration panel to delete all data.