The screen shot you posted is from Words with Friends and it will also come from any other app that uses the same Ad network. There are only about 8-9 ad companies that all apps use to provide ads for their apps and many of the same ads are sold to those ad companies. The same ad has also been seen on Zen Color as well.
If you close Safari with that page on the screen and open Safari again, it will open to that same page. You need to navigate away from that page or close the tab to remove it. The ad is going to have to be removed from the ad network that is providing it and the best way to get that done is to complain to the developer of the game. They get paid to present that ad to you, but without you playing their game, they will not get paid, so they have an interest to resolve the problem.