Well, i think you will have to remove the guppy pronto and keep it in a seperate tank. Once she heals, try letting her rejoin her old tank mates. If it still continues then she has to be separated for good.
MOst of the fish are attacking her because they know that she is weak and cant fight back. Just like humans , we like to pick on the weaklings, fish do the same thing as well.
Hope this helped