Many settings are stored in hidden sub-directories in your home directory.
In Linux, the name of hidden files/directories starts with a dot. I.e. the config data for KDE are most likely be stored in
.kde
The normal list command
will not show the hidden files by default. Only with the option
-a will hidden files/directories be listed as well.
This is not a bug, this is a feature. Settings of other applications are stored in the same way: Gnome settings in
.gnome, settings from gnupg are stored in
.gnupg, the personal settings of ssh can be found in
.ssh, to name a few.
Simply delete the
.kde directory and the settings are gone.
Note: First, I don't know if you can delete the kde settings while kde is running, i.e. use a terminal while kde is not running. Secondly, even though I am reasonably sure that kde will create a new default settings in
.kde, don't delete
.kde, but rename it, e.g. to
.kde-old. This way you can go back to the old settings if something goes wrong.