They already can limit who can respond to them on Instagram and Twitter.One thing I have noticed from all this is that no one understands social media. Nearly every single person in the world can make a post and in lots of cases (because the celebrity chooses to) can speak to high-profile people directly. Now I feel like these companies have an easy ride because of the libel exemption in America. Can you imagine putting up a community whiteboard outside your house that contained some of the stuff on social media?
Besides all that, the comments are obviously wrong but when anyone, in whatever state, can make a post to the whole world and to the person themselves, I am not sure what can be done apart from limited who can post. I don't know why footballers aren't calling for that. I am not sure what is being called for in the practical sense.
Most of them probably don't know that they can, or for other reasons decide not to.