Using PPA Purge:
ppa-purge can remove a PPA and downgrade all the packages you've installed from that PPA to the newest version in your other repositories. This is the cleaner way of removing repos.
sudo apt-get install ppa-purge
If you know the PPA address:
sudo ppa-purge ppa:<the PPA to purge>
sudo ppa-purge ppa:fcitx-team/nightly
If you don't know the PPA address:
You can also purge repositories that don't have the address in the correct format yet, by changing it. Change it from:
sudo ppa-purge -s <ppa_host> ppa:<ppa_owner>/<ppa_name>
Example: If you want to remove WineHQ repo.
Find the correct file:
ls -al /etc/apt/sources.list.d/
Get the address from that file:
Change the address from:
deb https://dl.winehq.org/wine-builds/ubuntu/ bionic main
sudo ppa-purge -s dl.winehq.org ppa:wine-builds/ubuntu
You can find a bit more info using
Or you can go to Software Sources on the Ubuntu Software Center Edit menu, enter your password, go to the Other tab, look for the PPA you wan to remove, click remove and close, it will ask you to update the repos and done.