Ubuntu

Removing repositories

From the terminal

sudo add-apt-repository -r ppa:<ppa-to-remove>

By editing sources.list

Old Ubuntu versions

Remove repository from /etc/apt/sources.list.

sudo nano /etc/apt/sources.list

Modern Ubuntu versions

Remove the <ppa-to-remove>.list from the /etc/apt/sources.list.d directory.

sudo rm -f /etc/apt/sources.list/<ppa-to-remove>.list