My gut reaction is to flag the heck out of this answer: https://ell.stackexchange.com/a/13144/50
However, I have to concede that the site being advertised is on-topic. (No clue about quality, as I have no wish to actually visit it.) Also, the post fully answers the question before it gets to the advertising, so it's not blatant spamming.
I don't know if I've encountered anything like this on any other site. I'm not seeing anything about link-dropping or advertising in our FAQ. (Aside: could we make the FAQ any harder to find? What's wrong with a link that actually says FAQ? Sheesh. But I digress.)
What should we do with an answer that includes advertising?
Nothing, as long as it's actually an answer, not just a link.
Edit out the advertisement portion.
Flag as spam.