Mary Ann,
 
There are two things you need to do.  The first is to make the
owner/apartment manager do an inspection, noting the pre-existing
condition of the areas the neighbor's little fuzzies have been using
as toilets.  You don't want to be responsible for someone else's lack
of training!  Make sure that you stress that your kids are litter box
trained, as are most fuzzies.  Otherwise, the apartment may become
unfriendly to ferrets.
 
Second: Try Nature's Miracle Cleaner/Deodorizer (available at pet stores,
Petco for one) and/or one of the Oxygen cleaners (such as Oxiclean).
Depending on the surface you are dealing with, you may need either or
both.  Don't forget the edges of the walls.  Any of the Oxygen cleaners
are good for rugs.  The Nature's Miracle is supposed to be good for all
surfaces.  I know a lot of people who swear by the Nature's Miracle.  (I
do dog/cat food demos at Petcos on the weekends.  I talk to a lot of pet
owners about this.)  If you are dealing with a truly hard surface, like
concrete, then the only thing to use is bleach solution.  DON'T use
bleach on wood or carpet, unless you intend to pay for the flooring you
ruin!
 
Good luck retraining your fuzzies once they notice that someone else has
been using the corner.  It took me nearly a month to retrain mine after
their "cousin" stayed here while his family was at Disney.  Take heart,
they can be retrained, even if they slip.  Just treat them like a new
puppy.
 
Pick them up and place them in the litter box when they back into the
corner.  Praise and reward them for using the litter box, especially the
outside-the-cage litter box, but don't forget to tell them they are good
boys or girls for using the litter box in the cage too.  (Both of my boys
will go back into the cage to use that litter box if it's closest during
playtime, rather than err in the corner.  Of course, that's because
they're hoping for a treat!) I immediately spray Lysol directly on the
accident spot once the mess is cleaned up.  It keeps them from going back
to smell the area, and imprinting that it's a toilet spot.  However, the
Lysol won't work on old stains.  It only works as a short-term deterrent
during training, as well as sanitizing the spot.
 
Fuzzies aren't hard to train.  You might want to speak to the neighbor
about training his to use the litter box in his new place.  Maybe someone
else won't have the problem you face when he moves next time!
 
          GEC @};~
 
"Don't smile at me like that!  That's not even a real smile; It's just a
 bunch of teeth playing with my mind!"  Lt. Templeton Peck
[Posted in FML issue 4535]