In the tap left, there is an arrow to return back to the app that opened the page you are seeing. It will continue to come back whenever that ad displays in Paint by Numbers. If you close the web browser with that page on the screen, it will show it again the next time you open the browser, so you would need to navigate off of that page.
Paint by Numbers uses the same ad network that many other apps also use and with enough complaints, this ad will be removed. The same ad shows for those with the Words With Friends and Zen Color apps. It is going to take complaints from users to the developer of the game, as they know without the users, they would not get any money from the ads.
The makers of Words With Friends have already notified their customers, that they are aware of the ad and working to resolve the problem.
https://zyngasupport.helpshift.com/hc/en/63-words-with-friends-2/faq/21160-why-am-i-seeing-a-suspicious-ad/