What you said
Sit the girl on her own whenever she causes problems. Explain why, and what she can do to demonstrate she is ready to work with the rest of the class. You could also contact the parents and invite them in. The horseshoe shape works well in secondary. For chatting and silliness, lay out your expectations and the consequences for breaking them. Then follow through every time.
Beware the horseshoe with chatty children. I taught some talkative Italian kids using this seating plan and the ones sitting opposite each other had a whale of a time abusing each other and cracking jokes across the classroom.
On day two, I put them into groups and gave points to each table with a small prize at the end of each day. The quieter children shut the noisy ones up so they could get the prize.
The expert view
Are rows and columns really not possible? Avoid a horseshoe if at all possible - it is an invitation to distract. Perhaps you need to turn up the volume on the sanctions. If they misbehave, they need to be kept in to do something decidedly unfun, such as copying, sitting in silence or tidying. Remove the chatterboxes from their peers if they persist. Have a zone ready to remove them to, whenever they disturb the class. Children hate being isolated from their peers, so if you are consistent they should get the message. Could you have partial rows with a curve at the end? At least then you could have areas that were mono-directional.
Tom Bennett's latest book, Teacher, is out now, published by Continuum. Post your questions at www.tes.co.ukbehaviour.