Would it be possible to add a button before the submit post or the submit new thread buttons that asked something to the extent of

Will your post or thread benefit racing in a positive manner?

[] Yes [] No

I understand this would not be fool proof but it may detour some of the negativity.

Another option would be a series of say 3 questions such as mentioned above. The questions would maybe give members a few moments to ponder the negativity they are about to unleash. Kind of a "cool down" period.