Ben Werdmuller posted a quick blurb about entirely disabling website notifications in your browser of choice, and I’m right there with him in not ever granting permission for a website to send me notifications. And while he covered Chromium- and Gecko-based browsers, he omitted Safari and Webkit-based browsers.
Fortunately Apple doesn’t hide this in hidden advanced settings panels. It’s in Safari Preferences → Websites (tab) → Notifications. Delete any entries you may have previously granted (or don’t) and uncheck the checkbox at the bottom.