You will need to remove the PostgreSQL source from your apt list. Fortunately, it's not too difficult to do via the Terminal. Here's how you do it:
Open Terminal (if it's not already open)
Change directories to /etc/apt/sources.list.d:
List the files in that directory:
You will see something like this:
drwxr-xr-x 2 ...
Where possible, it's recommended to not use PPAs at all. Adding a PPA is essentially giving the owner of the PPA root (administrator) level access to your system. It is possible for them to maliciously or accidentially replace system packages with broken/unpatched/malicious versions.
PPAs by trusted vendors are probably fine (Ubuntu, Libreoffice, etc), ...