Giant Schnauzers like any other big dog are very destructive when bored. They need a lot of exercise and stimulation. You might consider an outside kennel, when you are out, lave them lots of chews, toys etc and give them a lot of exercise. Can you get a dog walker or soemone to break the day. The other option is caging them in the house when you are out , but you will need lage cgaes as they need to be able to stand up and turn round.
They are not yet mature and should grow out of it but unless you break the cycle, it will become a habit and you'll never stop it.
From personal experience of other dogs, none of the pepper paste or anything like that works,.....distract or remove them from the chewing areas is the only way.