It's possible but here is an important "but":
If what you want is to fix the sorting to an other field like "title", "replycount", "views", "postusername", "voteavg". apply the hack below. But if you want to sort according to thread start date, no it's not possible with this small code change.
Anyway here is your hack:
Edit forumdisplay.php, find
PHP Code:
if (!isset($sortfield)) {
$sortfield = "";
Before that add this line:
PHP Code:
if {$forumid==1) {$sortfield = "title";}
if {$forumid==2) {$sortfield = "views";}
if {$forumid==3) {$sortfield = "postusername";}
what does it do?
It fixes the sortfield to "title" if forumid is 1, "views" if 2, "postusername" if 3.. Forum readers wont be able to change the sortfield for these forums anymore.
Not tested but should work..
Enjoy..
Logician