Well, A quick web search came up with a few ideas...
Prevention :
Bracken fern spread through spores, which are located under the leaves. When they ripen, the spores drift off like dust and pass through the air or travel in water until they find a good location. The spores cling to machinery,
boots and pets that move through infested pasture. Preventing the spread of the fern requires careful cleaning and sanitation, so the spores are not brought to uninfested areas. Machinery may have sections of rhizome in the lower parts that could
root and cause infestation.
Nonchemical Methods:
Bracken fern is such a tenacious plant that even fire won’t remove it. In controlled burns, it was found to be the first species to recover. Deep cultivation in the
course of 3 years has some effect, because the rhizomes are dragged to the surface where they dry out and die. Slashing removes the green tops of the plants and they eventually starve since they cannot produce carbohydrates. Rolling crushes the growing part of the plant and robs it of energy-producing photosynthesis, but it is not a efficient control.
Cultural / Habitat :
The best way to prevent bracken fern from getting established
is to have densely vegetated land. These ferns are very opportunistic
and maintaining a good ground cover of desirable vegetation
does help keep the spores from germinating.
Manual / Mechanical :
Pulling or mowing bracken fern in mid-summer can lower
vigor by depleting
energy reserves. Cutting in early summer,
allowing the rhizomes to regenerate a second crop of fronds,
then re-cutting will deplete the resources of the rhizome much
faster than a single cutting. Mowing will also reduce spore
production, helping to prevent spread, and allow more light to
reach the grass.
And an extension paper. ( as usual it talks about herbicides which are not used in
permaculture)
But the other info helps us to understand the plant and may give clues to help fight it, such as it not liking water. Maybe if you irrigate it heavily it might die back?
http://extension.usu.edu/rangeplants/htm/western-brackenfern