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:whatever/ppa

Note that this will uninstall packages provided by the PPA, but not those provided by the 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.d directory.