As a side note, the reason revoke usage on *.* from 'phpmyadmin'@'localhost'; does not work is quite simple : There is no grant called USAGE.
The actual named grants are in the MySQL Documentation
The grant USAGE is a logical grant. How? 'phpmyadmin'@'localhost' has an entry in mysql.user where user='phpmyadmin' and host='localhost'. Any row in mysql.user semantically means USAGE. Running DROP USER 'phpmyadmin'@'localhost'; should work just fine. Under the hood, it's really doing this:
DELETE FROM mysql.user WHERE user='phpmyadmin' and host='localhost';
DELETE FROM mysql.db WHERE user='phpmyadmin' and host='localhost';
FLUSH PRIVILEGES;
Therefore, the removal of a row from mysql.user constitutes running REVOKE USAGE, even though REVOKE USAGE cannot literally be executed.