Bitter Apple works by tasting *BAD*!  The problem with spraying directly
in/on a ferret is that it is very potent and provokes a very strong negative
reaction.  I once used it directly to separate 2 hobs who had broken out of
their cages and were determined to do each other in.  The reaction was quite
unpleasant to watch, and I would never do it except to prevent serious harm.
Also, spraying could get it in their eyes, which would be very painful.  I
had much better luck teaching a biter that ferret necks don't taste so good
by first spraying a bit on my fingers, then petting the back of the bitee's
neck.  This allows you to control how much the fuzzy is getting and where.
The biter gave up after a week or so.  Good luck!
 
Rob McNish and the Gang at McFerret Acres
[Posted in FML issue 1946]