I'm not going to name them, but I'm sure Blockland is hackable. It may be hard, but like I said, every program is hackable.
And why not? It would have taken no longer than that post. It sounds like you just don't want to be wrong.
I'll just use the situation in the OP as an example.
First off, sure there are few ways to get someone's key, but the ones that actually have any likelihood of happening involve user stupidity, and wouldn't be considered hacking.
Second, if someone else did have his key, there would be a join message when the 'hacker' joined
Of course people already realize this so go ahead and give me another example