Or from the terminal -

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

Or, edit the /etc/apt/sources.list to find your ppa there, and remove from there manually (or automatically).

Or, remove the ppa file from /etc/apt/sources.list.d in newer versions of Ubuntu.

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

via:WebUpd8

If you know the PPA address:

sudo ppa-purge ppa:<the PPA to purge>

Example:

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:

to

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:

cat /etc/apt/sources.list.d/dl_winehq_org_wine_builds_ubuntu.list

Change the address from:

deb  bionic main

to

sudo ppa-purge -s dl.winehq.org ppa:wine-builds/ubuntu

You can find a bit more info using man ppa-purge.

Via GUI:

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.