How to delete a MySQL user
If you wish to delete a MySQL user, login to your server via SSH & access MySQL prompt.

Select the database mysql & follow the steps below. Here, the database is mysql. Replace the database name as desired.
mysql> use mysql;
mysql> DELETE FROM user where user=’username’;
mysql> FLUSH PRIVILEGES;
Put the username of the person you want to remove in place of “username.” Additionally, choose the appropriate table where the user records are kept. This must be executed as the root user or as an account with the necessary grant table permissions.