Tear things up right in front of you? Or when you're not home?
If it's the former, then the best thing to do (aside from putting things out of reach in the first place) is to let the dog know it's not an appreciated activity with a loud "Uh uh"
followed immediately by an instruction to "leave it". Assuming, of course, that the dog has been taught the leave it command. When the dog leaves it, they must be promptly rewarded for complying, then redirected to a more suitable activity. After which, you curse yourself for not putting things out of reach, and put them out of reach
If it's when you're not home or don't see it happen, there is no possible way to effectively discipline the dog. That is, any discipline will serve no purpose other than to make you feel better. The dog, conversely, will be in total ignorance of why you're mad. They think in terms of cause/effect - and just as it is only ever effective to praise at the moment the dog is doing something you want, it's equally only effective to scold at the moment the dog is doing something you don't want. If you doubt it - imagine how far you'd get trying to teach 'sit' if you didn't praise at the relevant time, but came along 10 minutes later saying "hey, remember that sit you did 10 minutes, 30 minutes, an hour ago? Well, it was really good. Here, have a cookie". LOL - without doubt the dog would happily have the cookie. But there's no way in the world they could realise it was a reward for the sit, nor could it help to teach. Well, it's no different with punishment, signs of displeasure, redirection or whatever. Those *must* take place at the time the dog is doing whatever they're doing wrong. If you miss it, too bad. There's nothing that can be done except (a) to stop giving the dog the opportunity to do the 'bad' thing in your absence, and perhaps (b) trying to set them up to do it whilst you're there so that you can implement the above training.
As for the exercise thing - going out in the yard is not enough. Not unless you're out there too, anyway, engaging with her and giving her some good hard exercise. A boxer of any age needs to run for at least an hour daily, and adolescents for 2-3 times that much. If you don't give enough exercise (mental, as well as physical) then you end with a bored and frustrated dog that behaves badly. Really, it is amazing the difference proper exercise and mental stimulation can make.
