It will get more severe if u don't take action. there's no permanent fix but u can try this:
Reset advertising ID
Clear browser history and cookies
Wipe browser data and cache

It will stop the pop-up for a few days until it reappears again, but at least u'll have a piece of mind for a few days.

Update

After some digging, it looks like it was specifically targeted to certain devices, on the Mobile section only.

We have blocked some of the ads which could be used to push the pop up (thanks google for NOT filtering malicious ads)

Do let us know if its still appearing.

Some improvements..

[attachmentid=7419789]

Popup blocked, previously the ad still managed to defeat chrome popup blocker..

Can you try to clear/force close your Chrome browser and see if it still happens, i can no longer replicate the issue on my s7 after blocking out the malicious ads from adsense.

try use chrome or use adblock browser ?
if rooted just install adblock apps on your phone

looks like they added more ads, and google still isn't blocking them.

Blocked the new ads, do let me know if it still happens.