There is a good reason to be negative a priory in real live. Consider you start a thorough discussion on any problem that you encounter during the day. You wouldn't be doing anything else and die of hunger. In my opinion, people must be negative and quickly reject stuff that does not make sense to them. Forcing them to discuss about stuff that don't make sense to them is waste of time for both sites. For this reasons I would simply ignore negative comments and not ban them.