Hi Laura,
There's really no quick ways to do anything regarding the body. It has it's own sweet way to manage itself. Obviously if you don't work those muscles, they will atrophy in their own good time.
Are you on HT at all?
Sorry I couldn't be more helpful. Maybe for the time being your clothes fashion will have to hide your broad shoulders.
Speak to you as soon as I post my Christmas cards.
Huggs
Catherine