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

The DROP USER statement removes one or more MySQL accounts. It removes
privilege rows for the account from all grant tables. To use this
statement, 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 user name part of the account name, a host name
part of '%' is used. For additional information about specifying
account names, see [HELP GRANT].

With DROP USER, you can remove an account and its privileges as
follows:

DROP USER user;

URL: http://dev.mysql.com/doc/refman/5.1/en/drop-user.html

