make some hack, that have "reply counter".
make "reply limit" to forums with enable \ disable option.
when the files are "checking" the reply (number of letters + title etc') add "reply validation" to check how many replies he made today, and how many replies he have left. add to that "date checker" so it will check the date today. meaning grab the time now (TIME) and check the date, compare it to the date it have in the db (for replies) if its the same, check for replies validation, if its older then today, get the time now into the data base, and go for reply validation.
for example:
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i know it sounds maybe hard, and tricky, but read it a few times, and its easy, if you have some basic coding skills you can make it with no time.. but if you dont have coding skills i suggest you to try to read some other guys hacks, and get the main idea, it aint that hard (kinda fun tho).
if someone else is willing to make it for you, id be happy to help him out (with coding, fixing bugs etc').
i wish i had time to give you even the basic coding, but i really really dont

Sorry, and i hope i helped you out in some weird way

)
Fury