I know what you're going through!  I have found that Oxy Clean (the
crystals that dissolve in water) work great for eliminating the scent.
It hasn't stopped my little ones from going in the same area, but it does
get rid of the scent.  I wish I had good advice on retraining, but I've
been having trouble myself since moving with my fuzzies.  (they've even
gone ON bedding when I try to block spots!)
 
If you can't stop them from going I would buy those disposable puppy pads
and put one in that dresser drawer.  They're absorbent, so the ferrets
can go on them several times during the day, and they have a plastic
liner that prevents leaks onto what's underneath.
 
Hope you find something that works for you!
Risa
[Posted in FML issue 4815]