How do I destroy postfix on mac os x

sudo rm /System/Library/LaunchDaemons/org.postfix.master.plist will prevent postfix from starting at launch and being daemonized (if it has been installed in a usual way). This will effectively prevent postfix from running. However, removal of all files and config is another matter. If you installed using a package manager then that’s the way to remove it. Otherwise you have to search your computer and remove postfix-related files.

The unload comando just stops a service from running but, as you have noticed, starts again at boot because the plist file is still present.

You probably don’t need postfix on your Mac. It’s mostly for use for sending mail from a server (smtp server)—Gmail, yes, does that for you. You might have installed postfix so PHP can send emails, but for development you could try a faux postfix that logs instead of sends like Mailhog.