This is a cautionary tale to everyone with a laptop that they put a password on the system so that when it goes to screensaver or goes to sleep, it requires a password to get back in. Since Justin didn't have that, and he has DropBox on that system, he has data that he'd like to delete.
DropBox doesn't offer a remote delete capability, but the chatroom offers a link on how to do this. Unfortunately if he already unlinked the account, then this won't be possible. He should call DropBox support though and see what they could do for him.