Uninstall Postgres

$ sudo apt-get --purge remove postgresql

Now check what other postgres packages are installed

dpkg -l | grep postgres

This will list any other packages still installed, which can be removed using this command where {package name} is the package still installed

sudo apt-get --purge remove {package name}

Now delete all remaining remnants

$ sudo rm -rf /var/lib/postgresql/
$ sudo rm -rf /var/log/postgresql/
$ sudo rm -rf /etc/postgresql/
$ sudo deluser postgres
$ sudo delgroup postgres