There should really be a "Report this advertisement" button for ads that are inappropriate from the ad network. I just had an advertisement on here that had an animated girl in a maid costume with two buttons saying "Molest" and "Seduce". This is clearly not an ad that should be showing up for anyone, especially on Duolingo, but aren't any methods to tag them for review at the moment. Please consider adding this option soon.
As a temporary solution, please take a screenshot of these cases when you see them, and report them here. I was surprised to see the staff respond quickly when I reported a troll, so maybe they actually pay attention here as well, as Mr. von Ahn said:
We also try hard to block any ad content that would be considered objectionable by anybody learning a language (if you do see anything inappropriate, please report it to email@example.com with a screenshot).