How can I train my ferrets to behave?

I have two ferrets, my oldest is probably going on 2 years, and my youngest is barely a year old. Me and my Fiance just bought a brand new couch, its pretty nice.. its almost a leather-like material.. bad idea I know.. my ferrets are trying to eat it! This has been a nightmare. Is there anyway I can train them not to eat it? It's frustrating because sometimes she'll get in trouble and I'll scruff her and she starts all over again out of spite. My oldest also has a bad habit of using the bathroom in corners... and when I try to stop her, and put her in her litter pan, she REFUSES to go! I will even lock her in her cage until she goes, and she will go to sleep, or stay in there for an hour, and as soon as I let her out, she tries to squat in the same corner!...Help.. I'm getting frustrated. I love my babies but I'm having a hard time training.. the youngest is pretty good about the litter pans, but shes bad about following and doing as my oldest does.