if you can make it so that after 10 replies, they have to either wait 30 seconds before submitting again (or put in a captch, which ever is EASIER for you.
once they wait 30 seconds, (or fill in captcha), they can post fast again.
it has to be time related. for example, if they post 10 replies in 10 hours... it wouldnt make sense to force them to wait (or put in captcha)..
the condition is that, if they post 10 replies in 30 seconds.. then trigger the wait time (or captcha)