Ben Marcus mentioned using vinegar on those favorite "thinking" spots...
my experience... it works temporarily and must be refreshed within a day
or two (if I'm lucky, three to four days) as the vinegar smell begins to
dissipate.  Unfortunately I have one ferret who loves the taste of
vinegar... *sigh*... just when I thought I had the perfect solution.
 
Some additions to using the vinegar which help to keep its odor
neutralizing state is the citrus based orange cleaner.  It turns white
when added to water.  It tends to leave a residue behind that they don't
care for and then eventually get use to.  I have been alternating the two
and it seems to keep the ferrets away from trouble areas but again only
temporarily as ferrets will go where they want to.
 
Betty and Her Blur O'Fur
[Posted in FML issue 2633]