It's even easier than that. Instead of sliding the notification off the screen, slide it ever so slightly to the right. It displays this little gear thing, and you can block all notifications for that app from right there. I have a stock Nexus phone so YMMV.
This took me several minutes to figure out because I had a mental blind spot to "Sound & notifications" due to the fact that the option has a speaker/volume icon. I made the mistake of trusting the icon as a sincere shortcut for understanding the text it's supposed to represent.