How to block someone from seeing anything about you on Facebook?

It hasn’t gotten this far yet, but I’d like to know just how to do it if it comes to it.
Also, I have another question, but I’d better explain a little bit here first.
Girl A is using me as a messenger to my friend, Girl B. They knew each other last year, and something happened, and ever since then there’s been a…feud. So now I’ve been put in the middle of this because I’m a mutual friend (however, Girl A is most definitely not my friend). I’ve deleted her from my friend list, and I might need to completely block her from seeing anything about me (my first question). But, she might ask her brothers or sister to see if they can find me, or even break into their accounts and find me herself. How do I explain it to them that I have to block them so Girl A doesn’t keep bugging me?
I don’t think she would go so far that I would have to get anybody involved (except my parents, of course I’m telling them what’s going on).