There is horde-remove-user-data and the Clear Data buttons in the user 
administration.

This is a support question, not a bug report.

Please use the mailing lists to ask for support.

http://www.horde.org/mail/ contains a list of all available mailing lists.

This is interesting. It seems that this is only happening to those who 
have their preferences manually cleared for testing purposes, like 
below.

If this is not the correct way to clear a user from the database, then 
what is the preferred method? Are there other rows in other tables 
that need to also be removed to get a "clean slate" for a user?

Is there a guide for what rows to DELETE from what tables to 
completely remove an account from the database? I would find this 
useful, as I routinely have to delete customer accounts and want to 
make sure old data doesn't just sit around.

This is happening because the default calendar already exists (all the 
SQL statement does is remove the preference of which one to 
use/display). Since it already exists, it does not have to be created 
(and automatically selected).

It is behaving correctly. The prefs are checked to see which calendars 
to select. Since you emptied those preferences, none of them are shown.

Cannot reproduce. I cleared the user data completely and logged back 
in. The default shares have been created and selected, in all apps.

New users do not have their default calendar or task lists checked 
when they first login, and therefore their events and tasks are not 
displayed to them without manual intervention. This issue was first 
reported to me by others helping me test. I can replicate the issue by 
clearing out all the prefs from the horde_prefs table for my testing 
account. E.g.:

mysql> DELETE FROM horde_prefs WHERE pref_uid="testing";


After I do this, and logout and log back in, the default calendar and 
task lists are no longer displayed in Kronolith, and have to be 
manually "checked".

I'm using the latest Horde and Kronolith, which right now are Horde 
5.2.8 and Kronolith 4.2.11.