Some people simply cannot be dissuaded from occasionally posting inappropriate content. If they do it all the time, you might want to de-friend them. But it might be only occasional, and de-friending may have other real-life consequences (if only to your social life).
What kind of inappropriate content? The canonical example, I think, is for someone to tag you in a photo where you are doing something against the rules or otherwise inappropriate. It doesn't even have to be illegal; I've read a number of stories (and heard some personally) of elementary and high school teachers getting in trouble for simply being in a photograph where alcohol was also present. Even if they're over 21 and so is everyone else in the photo. I think it's horrifying that school principals and school districts are able to have that kind of policy, but some of them do.