On really old servers (you can skip this part if mysql version is > 5.0.2):
first drop all privileges associated with the account to be deleted:

show grants for 'user'@'localhost';
show grants for 'user'@'%';

revoke all privileges, grant option from 'user'@'localhost';

show grants for 'user'@'localhost';
show grants for 'user'@'%';


After that, or on newer servers, all you need to do is:

drop user 'user'@'localhost';