The voids and holes the fert climbs into is a natural activity for a
ferret.  Is what they do by innate behaviour.  You cannot train them
otherwise.  Bathroom vanities and kitchen cabinets are a good hidy hole
place for them, in my house have taken cardboard boxes, cut them to size
and made 'L' shaped covers and used a carpenters staple gun to staple them
over those holes.  Have put close meshed wire over the opening to the
fridge and closed off under the oven drawer and sides to keep them out
of there too.  Of course the washer and dryer had to be sequestered with
barriers.  They still find new holes time to time but is what they do.
You can't train a dog to not shed, and you can't train a ferret to not
check out a hole.  Or even make one if there isn't one!  Ha !!!!!!
 
Ferrets do not know right from wrong, and can't be trained to learn our
notions of right and wrong.  They do have their own set of 'right and
wrong' criteria but likewise we don't understand that ourselves anymore
than they understand ours.
 
I have trained all 4 of mine to not bite, well, at least 3 of them.. most
of the time.  Ha!  But that also is part of their behaviour.  As far as
climbing into a hole, why don't you make a place they can get into!  Take
a cardboard box, make a small hole they can get into, line is with a piece
of scrap carpet.  When it gets dirty, just throw it out and make a new one.
Will give them a place to go to satisfy that innate need.  Works well here!
 
Gordon, Byte-me, Nibble-ed, Kodo and Bud
[Posted in FML issue 3534]