Giving a neg rep to a post would not prevent discussion.
No but it would probably lessen discussion. Why articulate my opposing view point when I can just ping you with a -1?
It's not like negs and post quotes are mutually exclusive.
I would venture to argue that they are in that how many replies do we see to a post with tons of +1 saying "yea I agree"? Rarely do they match the number of +1's. The reputation system is a way for lazy people to "contribute" to the discussion and it's a low effort contribution that does absolutely nothing to advance the conversation.
It would simply provide another avenue of feedback.
We already have, by my count, eight ways to provide feedback. It should take a very compelling argument to add a ninth.
This leads to thread derailments and bickering. Is that the type of message board we want?
That's the message board we already have for a number of reasons unrelated to rep. Pull up any thread on more than its second page and there's a 50% chance it will have completely devolved into gifs.