If you added a PPA to your system, but you no longer want it, then you can easily remove it using the ppa-purge tool. To do that, first install the tool via the following commands:

sudo apt update
    sudo apt install ppa-purge

Now, if you want to remove a specific PPA, you have to have its full name (e.g fossproject/ppa). So for example, to purge the fossproject/ppa from your system:

sudo ppa-purge ppa:fossproject/ppa

And after that, the repository and its software will be removed from your system (if the PPA was just containing newer version of packages already installed before on your system and available in the official repositories, then they will be downgraded to the versions from the official repositories).