[Xenon]

you should find these lines in your editpost.php:

PHP Code:

if (!ismoderator($threadinfo[forumid],"caneditposts")) { // check for moderator
    if (!$threadinfo[open]) {
      eval("standardredirect(\"".gettemplate("redirect_threadclosed")."\",\"showthread.php?s=$session[sessionhash]&threadid=$threadinfo[threadid]\");");
      exit;
    } 


if you can find this, it should work.
but only users cannot edit posts anymore, a moderator can always edit posts in closed threads.