I read through this thread and didn't see this, so my apologies if someone has brought it up and I just missed it.
There is an easy way to keep your cats from going some place or shredding furniture even if you're not around, and it doesn't involve vigilance on your part every minute of the day - double sided tape.
You can buy wide pieces of it at the pet stores, although you'll pay a premium for it, or you can just buy double sided rug tape. (Used to keep rugs from sliding around.) This tape won't hurt furniture - it's not that sticky and it doesn't leave residue.
Apply the tape where ever you don't want the cats to walk or to scratch. The moment paw meets sticky the cat will get a really disgusted look and stalk off.
Combine that with a carboard scratching pad (like
this one) to give them a place that they CAN scratch and would like to scratch (sprinkling some nip into it helps!), and you shouldn't have problems with scratching inappropriately.