Just a couple of things.
The key words in your last post are "might be a youngish teen boy". People are often not who they say they are on line. What seems to be a young boy, could absolutely be a cat of a different colour. Verifications systems of age and profile pictures can be faked so easily online.
Typical online predator behaviour is to appear as some one of a compatible age, and conduct a relationship for a period of time- before asking for a meet or descending into inappropriate posting and asking the other person to respond in kind.
Additionally, a complete ban is often counterproductive. Much of a teens/tweens life is conducted online these days. Almost all social media users will be , at some stage exposed to inappropriate content. Teaching your child to manage risks, and to understand and empower them about how to handle is , is far more effective than completely banning it. Exposure to risk does not necessarily equate to harm.
Children who learn how to deal with crap like this are far better equipped than those who do not.
I'd suggest a big talk about online safety, background content in posts, wearing school uniform in pictures,the forever nature of the internet, privacy etc, I can go on, and on for ages.
But I would really suggest that a ban is not the way to go for an extended period of time. You will inadvertently teach her to hide such activity the next time it takes place, when she should come to you immediately. Social media has a huge effect on the young especially girls, and banning her is not teaching her how to navigate it safely.
Become her friend on all social media accounts, if she doesn't like it - she can't have the account. This puts you in a position of seeing all her activity.