Opencore 0.11 and earlier 

On older versions of opencore, you can go to the zmi and delete the user from the people folder and portal_memberdata.  This should also take care of the user in wordpress' mysql database.
 

Alternate Instructions

Or alternatively, on the zope debug prompt:
username = 'admin'
n = app.openplans
md = n.portal_memberdata
ms = n.portal_membership
md.manage_delObjects(['cabraham2', 'cabraham4'])
n.people.manage_delObjects(['cabraham2', 'cabraham4'])
import transaction
transaction.commit()


Then, in WordPress mysql:
mysql -u user_name db_name -p
<enter password>
in wordpress, you can look up this database information in wordpress-mu/wp-confi
(old build)
or in etc/wordpress/settings.php
(new build)

select * from wp_users where user_login like 'cabraham%';
delete from wp_users where id in (169,170);
delete from wp_usermeta where user_id in (169,170);