[Logician]

Here you go:

1-Edit forumdisplay.php, find:
-- cut --
switch ($sortfield) {
case 'title':
case 'lastpost':
case 'replycount':
case 'views':
case 'postusername':
case 'voteavg':
break;

default:
$sortfield='lastpost';
}
-- cut --

2- After that add:
-- cut --
$sortfield = "dateline";
-- cut --

This will sort all threads in all forums according to the first message date and user's sort field choice is cancelled.

if you want this feature for a specific forum (eg #4), use this instead:
-- cut --
if ($forumid == 4) {$sortfield = "dateline";}
-- cut --

For 4 and 5 and 6 use this:
-- cut ---
if (($forumid == 4) OR ($forumid == 5) OR ($forumid == 6)) {$sortfield = "dateline";}
-- cut --

This is tested and working..

Enjoy..