Task: delete user, that has been imported or created for testing purposes.
Assuptions: user has not logged, has not been registered to any trainings nor added to any student groups.
Solution:

This script has been tested by MWX engeneers.
This script is provided with no warranty.
Do backup your database before running this script.

1. Find out user_id of the user to be deleted by login name.

SELECT * FROM Users WHERE login = 'login_name_of_user_to_be_deleted';

2. Permanently delete user data

DELETE FROM UserAttributes WHERE user_id = <user_id>;
DELETE FROM TrainingRegistrations WHERE  user_id = <user_id>;
DELETE FROM Users WHERE  user_id = <user_id>;