What about a limit on how many comments a user can post in say an hour? I feel there is no need to be able to post 50 messages in 30 minutes.
If we can get fancy, use something like Leaky bucket -algorithm, https://en.wikipedia.org/wiki/Leaky_bucket
Set bucket size to 3, so user can post 3 messages in rapid succession. Bucket fills +1 every hour or maybe 30 minutes, so after those three rapid messages user can post one message per hour.