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.

Dismissing things that don't make sense probably isn't a good idea. Some of the most interesting things I've learned have been from working hard to discuss and understand something that didn't make sense to me at the time.

Also, there are often cases when someone has an idea or thought that isn't necessarily framed with the right wording. That doesn't make the idea invalid and it is often worth exploring to understand what they were trying to express.

These things is a 'waste of time', in my opinion. In fact, it is often the direct opposite: A wonderful use of time.