Comment 1 for bug 1614160

The right command would be e.g.
sudo ppa-purge -s archive.neon.kde.org -o user -p ""
sudo ppa-purge -s archive.neon.kde.org ppa:user/
But they probably fail, too.

You could tru my version of ppa-purge:
https://code.launchpad.net/~jarnos/ppa-purge/+git/ppa-purge/+ref/master