When I worked for the national trust we conducted experiments on the best way to control bracken in controled,fenced off bits on a side of a mountaine.I don't remember all the weird and wonderful things we tried but I do remember that the best organic method was to run over it with a roller(big one,a ton o'r two if memorie serves me correctly) a vibrating plate did the same job and was much easier to handle.
It seemed to make the bracken bleed and as it was expelling so much sap it really weakend it and after about 3 goes every 6 weeks it was gone.
There is a spray but I belive you have to be qualified to use it as it is highly toxic with no known antidote.