This isn't about not receiving messages from a person, but rather exit a group chat. But on the subject, is there a way of blocking someone i iMessages?
I'm not angry with Apple, but I feel you're being a bit of a fanboy. What's the point in defending the lack of a basic function? Especially with the argument that iMessage is "emulating SMS"? It reminds me of those defending the inability to send MMS with the original iPhone with "oh, you e-mail pictures, of course".
iMessage is an IM service, to include limitations of SMS when it's not needed is nothing short of retarded.