Roundcube
From Organic Design wiki
Deleting users
Deleting is done by user ID which is found in the users table, you can then delete the user as follows. In the following example we're using ID 1, you can also use the IN (ID,ID,ID...) syntax to deal with multiple at once.
DELETE FROM cache WHERE user_id = 1;
DELETE FROM contactgroups WHERE user_id = 1;
DELETE FROM contacts WHERE user_id = 1;
DELETE FROM identities WHERE user_id = 1;
DELETE FROM messages WHERE user_id = 1;
DELETE FROM users WHERE user_id = 1;
DELETE FROM contactgroupmembers WHERE
contactgroup_id IN (SELECT contactgroup_id FROM contactgroups WHERE user_id = 1);