One important configuration option is to ensure that login_lc is set to 2 which ensures that both the user and domain parts of logins are forced to lower case.
There's a script for deleting users
sudo -u www-data ./deluser.sh --host=localhost [--dry-run] USERNAME
If there's problems such as duplicate user rows, here's the manual way. 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 users WHERE user_id = 1; ; DELETE FROM contactgroupmembers WHERE contactgroup_id IN (SELECT contactgroup_id FROM contactgroups WHERE user_id = 1);
The last one is commented because we don't need it for OD, but some instances may need it