Good luck with this one - it's NOT easy to keep ferrets out of furniture.
In my experience, discipline is futile - although my ferrets tend to do
whatever they want anyway.  For my furniture, I've had the best luck with
duct tape - the heavy duty kind that is sold in hardware stores.  Don't
laugh until you've tried it.  No holes, nails or major construction
necessary.  Of course, the ferret will continue to dig at the tape for a
bit so you may need to replace or reinforce a few strips.  You also may
need to replace it after a few months when the "stick" of the tape wears
out but it really works.  After a couple of days, my two persistent and
spoiled ferrets gave up on trying to get into the couch.
 
Best of luck in your challenging endeavor!
 
Melissa Sullivan
[Posted in FML issue 3254]