Lots of my users say they're getting a popup saying you've won an iPhone or similar things, looks like those spammy redirects.

The site has been checked for all vulnerabilities and code changes and found nothing. Adsense was all taken off and the issues were fixed. Then adsense was put back on and within minutes people were complaining.

How can this be fixed? It's almost certainly dodgy ads in AdSense.

The problem is I have no idea what adverts I'm looking to block