Some of the Wikipedia mailing lists did this a year or two ago, adding a limit of 30 posts per person per month. Not sure it's had any significant impact on the community, but it has somewhat reduced the volume of a few really, really prolific posters, which is a slight improvement to the S:N ratio. (There exist mailing lists where the most prolific posters are also good contributors, but imo this is not true of the Wikipedia lists, so the cap has mostly excluded posts I find noisy/redundant.)

Interesting! I would rather use a one per day limit instead of 30 per month - this would force people to think another 24 hours about their reply and that could improve it :)