Plagiarism is a serious offence in the real world. A small community like this will pretty vulnerable to such a thing, because chances are there's not going to be enough of you raising suspicion. The only reason you did in this case was because the offender left pretty blatant evidence in his WiP. What would happen if he didn't? Would he get away with it?
Bigger communities (e.g. music, scientific, literary, creative) will always use software to check for plagiarism offences for submissions by default. Though i'm guessing the hive/contest host don't have access/don't utilize these tools?
Honestly, the best defense you guys have against situations like that is how well you know each other (e.g. deepstrasz - well known and talented, you'd expect something good from him, vs Forsakethis - basically a new profile) and how realistic an entry is (e.g. deepstrasz - a believable entry because although it's great, it's not perfect, vs Forsakethis too good to be true). You pretty much did this in the first instance, so just keep it up and move on