LOL good luck "banning" an teenager from anything. She's exploring her identity. Today's "emo" is yesterday's "goth." Sounds like her identity with this stereotype is pretty superficial. I was a happy-go-lucky teenager with a job, top grades in honors and AP classes, was involved in cheerleading, theater, music, art, student council, the environmental club, peer counseling and church and still liked the Goth look - black clothes (when not wearing my school uniform), heavy eyeliner, black lipstick, short dresses and combat boots, dyed my hair black (from dark brown LOL), etc. Oh yeah and the depressing music and the feigned ennui that went along with it.
What I would do is mostly ignore it. As long as her behavior doesn't change, she's just into the look and the music. It'll probably pass. Ask her in a non-judgmental way what about emo is appealing to her. If emo isn't a popular look where you live, it may be that she's trying to rebel against the more common looks for girls her age (preppy, slutty, gangster girl, etc.) and either separate herself from others that she doesn't like or likes the attention of dressing differently. If there are other kids who are into this look, she may be trying to fit in with them. Emo kids tend to think of each other as "deep" or mature.
My step-daughter and oldest son are both 15. They've tried on different looks and styles over the years. As long as they are covering up parts they're supposed to cover up and can clean up OK for church and important holidays, I don't care what they look like. They usually figure out on their own that they look ridiculous and get away from the extreme looks. My oldest son considers himself a "hipster" and last year, my SD had "scene" hair, which was basically a really, really, really bad haircut. I didn't say anything and she decided on her own that she didn't like it and hasn't tried anything edgy since.
Don't worry about this as long as it's looks and music only. If her behavior and grades and friends change for the worse, then you've got something to worry about.