"Positive reinforcement" sure is a creative way to give a good example however I don't think it's going to be happening. This would imply that the forum hires and endorses some kind of elite posters to give a good example. In a way, it would also mean that they endorse ideas that the forum should in fact keep a neutral stance on. Would this actually help free speech? Should those participants stay away from supporting controversial views? I don't think this complication would help the neutrality we've had so far here.

As of the "silent treatment". Reddit used to have this, it was called a shadowban and was exactly what shorena described. Although the system was functional it wasn't transparent and didn't work really well. In fact they changed the banning procedure to be more transparent and "correctional" just today. https://redd.it/3sbrro