I would be honest with her, and tell her her prior actions were hurtful. Explain that you would like to let her Skype her family for Christmas, but this is a one time thing, and any further interactions will be at your discretion. She has abused your trust, and will have to earn in back slowly.

If she apologizes and takes it like a grown-up, great. If she tries to make you feel guilty, doesn't apologize and blames anyone but herself, tell her "X place has Internet you can use for a fee," and leave it at that.

My 2 cents!