I would put fuzzy blankets, toys, food and water where you don't want him to go potty in. I usually cover the whole floor with blankets.
Depending on the size cage you have him in, you may need to put more then one litter box in the cage. I have a 4-6 levels cage and I put a litter box on the bottom and top levels and then surround it with blankets.
Some ferrets are stubborn though and refuse to use the litter box. I wouldn't use anything smelly to ward him off, trust me it only makes it worse because they want to cover up the bad smell!
The only other thing to do is to pick him up and put him in the litter box every time he is about to go in the wrong spot. If you don't catch him in time and he went poo on the floor, scoop the poop and stick it in the litter box, and then stick him in the litter box and let him sniff the poo so he knows that is where he is suppose to go. When he does go in the litter box give him a treat. But Watch Out, because they are smart and will start faking going poo and pee just to get a treat out of you! Mine did it to me, thats when I started watching them carefully and making sure they actually go poo and pee before giving them a treat.
Once your ferret realizes that he gets a treat when he goes in the litter box, he will be more then willing to use the litter box. Once he goes in the litter box 70-90% of the time, thats when you need to cut the treats back to every now and then when he goes instead of everytime he goes.
Good luck, and remember, some ferrets are just not meant to be litter boxed trained! I have one in paticular that refuses to go in the litter box despite how many times I stick him in there and give him treats. Luckly, he at least goes in the litter box when he is in his cage. But unfortunatly for me, I get to step in poo every morning!=P
Sampson is the only one though, out of three ferrets that is 40% litter boxed trained. I know some owners who can never get their ferrets litter boxed trained, inside or outside the cage. My other two ferrets(Buddy and Jessie) go in the litter box about 98% of the time. It is good to start them while they are young so its burned in thier heads. I think the reason why Sampson is so stubborn is because he wasn't mine since he was a baby, I adopted him from the vet and I have a feeling he isn't use to being outside his cage. He is also blind and his rear legs are weak, so between that and his age and his history, I guess it will be an endless battle with him.
Hey, let me know how it goes, and hey, if I get Sampson litter boxed trained outside his cage I'll let you know!=)