DROP USERuser[,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 user name part of the account name, a host name part of
'%' is used. For additional information about
specifying account names, see Section 12.4.1.3, “GRANT Syntax”.
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.
Before MySQL 5.0.2, DROP USER
serves only to remove account rows from the
user table for accounts that have no
privileges. To remove a MySQL account completely (including all
of its privileges), you should use the following procedure,
performing these steps in the order shown:
Use SHOW GRANTS to determine
what privileges the account has. See
Section 12.4.5.17, “SHOW GRANTS Syntax”.
Use REVOKE to revoke the
privileges displayed by SHOW
GRANTS. This removes rows for the account from all
the grant tables except the user table,
and revokes any global privileges listed in the
user table. See Section 12.4.1.3, “GRANT Syntax”.
Delete the account by using DROP
USER to remove the user table
row.
DROP USER does not
automatically close any open user sessions. Rather, in the
event that a user with an open session is dropped, the
statement does not take effect until that user's session is
closed. Once the session is closed, the user is dropped, and
that user's next attempt to log in will fail. This
is by design.
DROP USER does not automatically
delete or invalidate any database objects that the user created.
This applies to tables, views, stored routines, and triggers.

User Comments
So DROP USER does not automatically close any open user sessions, but does it prevent that user from opening new sessions in the meantime?
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