How do I remove an application from the notification center?

I somehow started receiving 'notifications' for Tom's Hardware after visiting their site. I went to the Notifications System Preferences Panel hoping to remove them, not just turn off the notification. I can't find a way to do this.

It's a little concerning that just visiting a website allowed a permanent intrusion on my system. I noticed a couple of applications in there I would like to remove as well. How do I do that?

My current OS is El Capitan 10.11.6