using ppa-purge (with/without -i) results in suggesting removal of over 100 packages I'm fairly sure ARE available in the standard ubuntu repository

First, manually delete the ppa using the following command:

sudo rm /etc/apt/sources.list.d/kxstudio*

Now, use "apt-pinning" to give priority to your current distro, regardless of package version number. First, create a file:

sudo nano /etc/apt/preferences

and copy/paste the following into the file replacing "trusty" with your current ubuntu version:

Package: *
Pin: release a=trusty*
Pin-Priority: 1001

Press CTRL + O and then press ENTER to save the file and press CTRL + X exit.

Now, purge the specific kxstudio packages:

sudo apt-get purge kxstudio-artwork kxstudio-default-settings kxstudio-desktop kxstudio-desktop-kde4 kxstudio-welcome

Then, to apply the downgrade:

sudo apt-get update
sudo apt-get dist-upgrade

modified from source: http://ispire.me/downgrade-from-debian-sid-to-stable-from-jessie-to-wheezy/