Itch.io will never place advertisements on any of your game pages. I'm fairly certain that your machine is just affected with adware - give it a good scan with a reliable antivirus and it should go away. I was wrong, see below.
Well, I tried using Windows Security Scanner and MalwareBytes along with MalwareBytes AdwCleaner, and while AdwCleaner did get something, it didn't solve my problem. I later thought of some other cause to my problem and decided to use the Playstation 4 browser to visit a free game page to see if the ad appeared on my PS4 too, and it did. My Playstation 4 can't have a virus because I've recently formatted it completely and this was pretty much the first time I've used the browser since, and I don't think many viruses can infect a PS4 anyway.