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

The DROP USER statement removes one or more MySQL accounts and their
privileges. It removes privilege rows for the account from all grant
tables. An error occurs for accounts that do not exist. To use this
statement, you must have the global CREATE USER privilege or the DELETE
privilege for the mysql database.

Each account name uses the format described in
http://dev.mysql.com/doc/refman/5.6/en/account-names.html. For example:

DROP USER 'jeffrey'@'localhost';

If you specify only the user name part of the account name, a host name
part of '%' is used.

URL: http://dev.mysql.com/doc/refman/5.6/en/drop-user.html

