Griefing anything, in general, is not okay. Even if it was just a few blocks, you should show a little more respect to other people's property. If you broke a few blocks, have the sympathy to replace those few blocks.
If someone asks you break down something, that's perfectly fine, but if it's an unwarranted destruction (as is your case), action will be taken to correct the issue.
To add, looking through someone's chest, unsolicited, is not okay-- taking something from someone's chest is considered griefing and it will not be tolerated.
