Girl Who bullied me and is emailing me now

She's not being rude in the emails or mean it's just she's threatened me off and on bullied me off and on and now she's emailing me through social media right now she's in one of her nice moods and being nice but one minute she'll flip as well so how do I tell her that she can't contact me through social media I tend to tell her in person instead of through email because I think that's the best way.