How can PPAs be installed or removed?

Add PPA by command:

sudo add-apt-repository ppa:whatever/ppa

Use the --remove flag, similar to how the PPA was added:

sudo add-apt-repository --remove ppa:whatever/ppa

As a safer alternative, you can install ppa-purge:

sudo apt-get install ppa-purge

And then remove the PPA, downgrading gracefully packages it provided to packages provided by official repositories:

sudo ppa-purge ppa_name

Anyway, this won't uninstall packages that were on the PPA but not on tha official repositories. If you want to remove them, you should tell it to apt:

sudo apt-get purge package_name

You can also remove PPAs by deleting the .list files from /etc/apt/sources.list.ddirectory.

Last but not least, you can also disable or remove PPAs from the "Software Sources" section in Ubuntu Settings with a few clicks of your mouse (no terminal needed).