I think it would be good to do a targeted posting rate eg. 30 posts per hour[1] or something of that nature.
If last 20 min when regressed predict under 30 posts in the next hour it should cost 0 karma to post. However, for every post/hour over the target rate should cost the user some karma eg.
The posting page should of course warn the user of the karma loss and confirm that they really want to post.
[1] 30 posts/hour stands for whatever target rate you want
EDIT:
it occurs to me you could apply a learning algorithm to learn what alpha should be based on experience of what yields the closest posting rate to the target rate. you could also reward people for posting when it gets slow.
Don't forget second-order effects. Your proposal encourages people to post quickly, to try to get in before the costs start kicking in. Encouraging people to crank posts out faster is probably not a net win.
I've noticed that trying to engineer communities isn't entirely unlike trying to secure software. You're not defending against straight-line attacks, you're defending against a hostile adversary with more time, resources, and collective brainpower than you, who will actively react to any move you make. The miracle is that any community ever works at all/any software is ever secured at all.
If last 20 min when regressed predict under 30 posts in the next hour it should cost 0 karma to post. However, for every post/hour over the target rate should cost the user some karma eg.
where alpha is a tune-able "cost" constant.The posting page should of course warn the user of the karma loss and confirm that they really want to post.
[1] 30 posts/hour stands for whatever target rate you want
EDIT:
it occurs to me you could apply a learning algorithm to learn what alpha should be based on experience of what yields the closest posting rate to the target rate. you could also reward people for posting when it gets slow.