You can uninstall or removes an installed 0install package itself from Debian 9 (Stretch) through the terminal,
$ sudo apt-get remove 0install
If you would like to remove 0install and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove 0install
If you use with purge options to 0install package all the configuration and dependent packages will be removed.
$ sudo apt-get purge 0install
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove 0install