This question may look like a duplicate of: How to uninstall postgresql on my Mac (running Snow Leopard) however, there are two major differences. I\'m running Lion and I\'m
I don't use the same version, but uninstall actions are the same: Looking for file uninstall-postgresql inside directory
/Library/PostgreSQL/9.6
then run it.
(Screenshot in macOS 10.13)
then
sudo rm -rf /Library/PostgreSQL/
to delete all unnecessary directory.