Ok, I had the same problem a while back and could not afford a new
sofa. Soooooo... What I did was go to Home Depot (or Lowe's) and bought
2 cans of Great Stuff' foam. It is a fast drying, insulation that
blows up when you spray it and dries pretty hard. It also comes w/a
straw-like applicator to get it into cracks. I pulled off the cushions,
and sprayed it into any areas where I thought they could wiggle their
little fizzbutts into, then let it dry thoroughly before letting them
out (24 hrs I think it said on the can - and leave the cushions off
till it dries, but don't mound it so much that the cushions won't fit
back into place but in case you do, you can cut it down w/a serrated
blade, but that's makes it a little easier for the ferret to dig at
as it doesn't have it's 'smooth' surface anymore and has some of the
'holes' or air bubbles that make it expand so's they can get a claw
hold on it.) Then we gave it a try. They found a few spots I missed -
and I even had to slit the bottom webbing under the seat cushions to
get one of them out - but I sealed up the hole AND the slitted area
too! Needless to say it was ugly w/the cushions off, but put back
together you'd never known and no more lost fuzzies! Oh, don't try
wedging towels and rags into those cracks - it doesn't work. They just
dig and dug till they get them out. The cement hard 'Great Stuff' did
the trick! I think they tried digging at it but after hardly making a
dent (some dried flakes and claw marks), they gave up. It's really
GREAT STUFF!! I know it's toxic AND flammable in it's liquid form, but
once dry, I have never heard anything bad about it. All the warnings
on the can are in it's liquid state. This stuff WILL make a mess if
you get it on something you don't want it on AND on your hands! Wear
gloves! It's hard to get off!!

Here's a little info on it.
http://greatstuff.dow.com/greatstuff/cons/faq htm#faq9

If anyone else has any other info that is is dangerous, please let
everyone know. I've used it w/great success for ferret digging under
cabinets too and no one got sick. All my ferrets lived to be over 6
and a few to 7.

Dawn and the gang!

[Posted in FML 5981]