Syntax:
DROP USER user [, user] ...

The DROP USER statement removes one or more MySQL accounts. To use it,
you must have the global CREATE USER privilege or the DELETE privilege
for the mysql database. Each account is named using the same format as
for the GRANT statement; for example, 'jeffrey'@'localhost'. If you
specify only the username part of the account name, a hostname part of
'%' is used. For additional information about specifying account names,
see [HELP GRANT].

DROP USER as present in MySQL 5.0.0 removes only accounts that have no
privileges. In MySQL 5.0.2, it was modified to remove account
privileges as well. This means that the procedure for removing an
account depends on your version of MySQL.

As of MySQL 5.0.2, you can remove an account and its privileges as
follows:

DROP USER user;

The statement removes privilege rows for the account from all grant
tables.

URL: http://dev.mysql.com/doc/refman/5.0/en/drop-user.html

