I was searching for the same mod a week ago but i wanted to lock it after 1000 total posts.
I came across something but since my forum is too new, i haven't reached that 1000 mark in a thread, yet.
Let me find that code for you....i'll also let you know if it works for me.
--------------- Added [DATE]1246554974[/DATE] at [TIME]1246554974[/TIME] ---------------
actually, i found what seems to be a better mod:
https://vborg.vbsupport.ru/showthread.php?t=196678