First, fire up MySQL in your terminal and login as an administrator account with:
mysql -u root -p
You will be prompted for your root password before being granted access to the MySQL monitor.
Now to grab a list of available user accounts on the system. Running:
select user,host from mysql.user;
Once you have spotted the account you wish to remove, run the following query:
drop user 'username'@'localhost';
Simple and nifty.
Related Link: http://dev.mysql.com/doc/refman/5.1/en/drop-user.html