How can I prevent my ferrets from chewing on wires?

What's worked very well for me is a combination of a few things.

I limit the amount of wires the ferrets actually have access to. The power-strip for the television is hung on the underside of the table with large command strips. The table is a little over two feet high and I don't put anything under it so the ferrets would never be able to jump/climb up to the wires. Gratuitous use of cable ties keep TV and console wires on the tabletop.

The power-strip cord is held up along the perimeter of the wall with command hooks, out of ferret reach.

The part of the cord they do have access to, around the outlet, is coated with this bitter spray. It's available at Petco and other chain pet stores. It tastes horrible (tried it on the car ride home; big mistake) and once the ferrets get a lick, they're usually very uninterested in whatever they just tried.

In general, I would not give my ferrets any toys made of rubber or similarly indigestible material that they could bite a chunk off of. Ferret kongs are a good example of what not to give them. They can bite off a piece, swallow it, and get a bowel obstruction. Ferret proofing is important.

Vigilant “ferret proofing” of the home is a must, being especially careful to remove all rubber and foam items that ferrets are particularly fond of.

The last part requires a bit of diligence. I watched my ferrets and every time they went for the wire, I would scruff them and drag them away from the wire. The dragging is something that ferrets will do to each other when trying to establish dominance. This is useful for training out any bad behaviors in ferrets (like nipping as the video mentions). You need to be consistent and make sure you get them every time they do it immediately after they do it, else they won't get the right idea. With enough time, it's possible to get them to ignore the wires completely.