This blog post explains very well:
(just replace 9.X by your version. e.g: 9.6)
A. If installed PostgreSQL with homebrew, enter brew uninstall postgresql
B. If you used the EnterpriseDB installer, follow the following step.
Run the uninstaller on terminal window: sudo /Library/PostgreSQL/9.X/uninstall-postgresql.app/Contents/MacOS/installbuilder.sh
C. If installed with Postgres Installer, do:
open /Library/PostgreSQL/9.X/uninstall-postgresql.app
Remove the PostgreSQL and data folders. The Wizard will notify you that these were not removed.
sudo rm -rf /Library/PostgreSQL
Remove the ini file:
sudo rm /etc/postgres-reg.ini
Remove the PostgreSQL user using System Preferences -> Users & Groups.
Unlock the settings panel by clicking on the padlock and entering your password.
Select the PostgreSQL user and click on the minus button.
Restore your shared memory settings: sudo rm /etc/sysctl.conf