Alrighty, I thought I had this problem solved with one of my two ferrets but I was wrong-- when I moved away from home I noticed he was using crevices and corners I couldn't see/reach (it was an old house...) yucky!

I got these two when they had just turned a year, I've had them for almost a year... so they're both nearing the age of two now.

One of them is fine using the litter box both in the cage and outside during playtime-- I reward him occasionally (really, every time I see him go to the litter box and see that he used it) with ferretone.

The other is fine in the cage using the litter boxes but once it's outside for playtime, he completely refuses to use the litter boxes provided. Originally, before I bought bigger litter pans for out during playtime, I had just moved their cage litter boxes out and let them use that-- or really, let the one ferret who successfully uses the litter box. 

They both use it in the cage, but the other literally completely refuses to use the litter box out during playtime. The logical response would be to place litter boxes where he does his business, and that was the first thing I tried- that's where the litter boxes are now placed, but now he picked two new spots that I simply cannot put the litter boxes in.

I have my living room blocked off from my kitchen and bathroom, they play here in the living room. It's spacious enough, but not too spacious so they get lost or anything-- and the spots he now chooses to do his business is either directly in front of the plexiglass I have as a blockage splitting the living room and hallway from each other-- so if I put a litter box there, I can't walk over it to get to and from the kitchen and bathroom-- and I'm afraid of them trying to use the litter box to attempt to jump the plexiglass-- and the other spot he chose is right next to or right in front of my computer. The computer is up against the wall- and my chair is basically the same, so putting a litter box there wouldn't really work, and if I put it next to the computer it would be under my desk-- where my feet are.

I try to catch him before he goes and I put him in the litter box- and we continue to do so, as he refuses and tries to leave it multiple times-- to the point where he literally cannot hold it in any more and does his business in the litter box. I ALWAYS reward him with ferretone if he successfully goes in the litter box (even with my help-- which, always is).

I tried sternly telling him "NO!" while scruffing him if he does his business not in the litterbox, I tried 'time-outs', I tried putting his nose near it and telling him no, (immediately after he does it, not any time later)-- I tried ignoring it (just cleaning it up) and when I catch him in time to force him to do it in the litter box and heavily rewarding him (good boy! good boy! + ferretone), I tried getting new litter boxes and not allowing the other ferret to poop in the second one (thinking maybe one "claimed" one of the litter boxes by doing business in it), and putting litter boxes where he was originally doing his business.

He seems to simply absolutely refuse; but he has no issue doing it in the cage in the litter box-- where there are two, one on each level-- which they both share, but when he comes out he just plain refuses. Like I said, he jumps out of the litter box and tries to run right back where he was gonna do his business (in front of the plexiglass and next to or in front of my computer) until I continually bring him back to the litter box and don't let him leave 'till he does this.

I should add- he also will try as hard as he can to do it while I am not looking or paying attention. He will go to the spot, and he will "speed bump" or lay there- and pretend he's not doing anything- but of course, I know better. And sometimes, I'll be in the middle of something-- so I'll look away for a second-- and BAM, he's already 1/2 way through his business. It's like a HUGE slap in the face! 

And usually, this is understandable to be a long process with patience with kits or moving away; but I've been doing this for nearly a year (about 2-3 months shy of it) and he just will not do it. It's becoming really discouraging because it's like-- yay, play time!-- oh, I have cleaned up 5 of your poos and 3 of your pees today and my whole roll of paper towels are gone! (And here in NYC, paper towels are hella expensive!)

But yeah, sorry for the book; I'm just at a total loss of ideas now.

TL;DR: 2 y/o ferret does his business in the cage's litter boxes fine; but when out for play time refuses- and I mean REFUSES- to use the litter box at all possible costs. Placed litter boxes where he went poo, avoids that now- got new litter boxes that are certainly spacious enough, still avoids it; tried assigning litter boxes to each ferret- still avoids it. Been trying to teach him for about a year. Still isn't succeeding. Help!

Edit: Boyfriend just suggested puppy training pads.... hm. What do you guys think?!