Are you sure you are 19?
You seem more concerned about telling us about your bf than explaining the problem. I'm betting you are more like 15. I be it is your mom's phone that she lets you use, so of course you would be concerned about the naughty pics sent by your bf. (Why would you have a phone, even if you are 19, but your mom does not so borrows yours? Really? I don't buy it.) First you say this girl is your friend, then at the end you don't like her. Which is it? And who the heck cares what she looks like or that she likes Justin Bieber?
Fact is, I don't buy into your whole post. I think it is poppycock. But, on the off chance it is not, I will answer your question.
what should i say to her so she can go away
Just say, "Go away." Block her on all social media. Block her on your phone (or your mother's phone, as the case may be.) That should do it.