Attention: the line numbers won't be the exact ones because I already did some other hacks on my code (probably like you).
I've added the numbers, so you would know if you've had to look at the beginning of the file or around the end.

edit your b2posts table:
add an extra row named post_comment, make it a tinyint(2), NOT NULL, default '1' (this way the comments will be on as default)

Open b2edit.php

look for the $query that says (around line 74):
$query = "INSERT INTO $tableposts (ID, post_author, post_date, post_content, post_title, post_category) VALUES ('0','$user_ID','$now','$content','".$post_title."','".$post_category."')";

edit this query like this:

$query = "INSERT INTO $tableposts (ID, post_author, post_date, post_content, post_title, post_category,post_comment) VALUES ('0','$user_ID','$now','$content','".$post_title."','".$post_category."','".$post_comment."')";

Look for another query looking like (around line 188):

$query = "UPDATE $tableposts SET post_content=\"$content\", post_title=\"$post_title\", post_category=\"$post_category\"".$datemodif." WHERE ID=$post_ID";

Edit it like this:

$query = "UPDATE $tableposts SET post_content=\"$content\", post_title=\"$post_title\",post_comment=\"$post_comment\", post_category=\"$post_category\"".$datemodif." WHERE ID=$post_ID";


Open b2edit.form.php (within b2-include)

Look for these lines (around 25):

		else {
			$form_ljupdate = '';
		}
		$colspan = 3;
		break;

Insert just before $colspan=3; this extra line:

$form_edit_comment = '<br /><br /><b>Comments on/ off</b>:</label><br /><input type="radio" name="post_comment" value="1" CHECKED/> On | Off <input type="radio" name="post_comment" value="0" />';

Look for these lines (around 119):

	<?php echo $form_pingback ?>
	<?php echo $form_ljupdate ?>


Add these extra lines after <?php echo $form_ljupdate ?>:

<?php echo $form_edit_comment?>
<?if ($action=="edit"){?>
<?
$query = "SELECT post_comment FROM $tableposts WHERE ID = $post";
$result = mysql_query($query);
$ar22=mysql_fetch_array($result);
?>
<br /><br /><label for="comment_aan"><b>Comments on/ off?</b>:</label><br /><input type="radio" name="post_comment" id="post_comment" value="1" <?if ($ar22['post_comment']=="1" || $post_comment==""){?>CHECKED<?}?>/>&nbsp;On&nbsp;|&nbsp;<input type="radio" name="post_comment" id="post_comment" value="0" <?if ($ar22['post_comment']=="0"){?>CHECKED<?}?>/>&nbsp;Off<BR>
<?}?>

Edit b2comments.post.php

Find (line 88):
/* end flood-protection */

just below, add:

$query = "SELECT * FROM $tableposts WHERE ID='$comment_post_ID'";
$result = mysql_query($query);
$ar=mysql_fetch_array($result);

if ($ar['post_comment']=="0"){
    die("Comments are disabled for this post");
}


Edit your template (index.php).
Remove the line that will insert the link to the comments. At my site it looks like:

<?php comments_popup_link("zero comments (0)", "what this one person thinks", "what are they thinking (%)") ?> 

It could be either comments_popup_link() or comments_link()

Replace it with:
Be sure you replace "b2posts" within the query with your own b2posts-table-name!
You can edit the comments_popup_link() function like you normally do.

<!-- starting of showing comments or not -->

<?
$post_id=$id;
$query2 = "SELECT post_comment FROM b2posts WHERE ID='$post_id'";
$result2 = mysql_query($query2);
$ar2=mysql_fetch_array($result2);
if ($ar2['post_comment']=="1"){?>
<?php comments_popup_link("zero comments (0)", "what this one person thinks", "what are they thinking (%)")?>
<?}?>

<!-- end of showing comments or not-->


That it!

If you've any questions or there's a bug? Mail me at snuggles@veel-in-een.nl

http://www.veel-in-een.nl
(weblog in Dutch, anything about everything)