Sounds like the same problem I battled about a year ago.  (Full post was in
FML# 1954 dated 6/2/97- "litterbox Woes & Ohs".)  Try the following:
 
Give up all hope of getting them to not use that area.  Clean it up with a
sponge or better yet a shampoo machine, let it dry and then put down rug
runner to protect the carpet.
 
Incredibly when I did that after 2 months of trying to get them to use their
2nd litterbox my ferrets started to use it!  Brats!  They just wanted for me
to give up.
 
One of the compromises I've made was to put the litterbox where they want
it.  Smack in the middle of doorway.  I never close the door-I have a
barrier in the doorway that I step over.  So I have to step over it AND the
litterbox, but as long as they use the litterbox and not the rug, it's an
agreeable arrangement.  Other fomerly favored litter areas were cleaned,
blocked and/or stuffed with bedding.
 
After more than a year of adjustment they now use the litterbox outside the
cage 95%+ of the time-it's been weeks since I've cleaned poop off the rug.
But for months the hit rate outside the cage was ZERO.
 
Once they use the litterbox outside the cage, you can try moving it and
putting bedding down in the doorway.  Try moving it just a few inches at
first.
 
-Ilena Ayala
Katie and Polly-litterbox sweet litterbox users
[Posted in FML issue 2364]