For that column, simply set:

$col["editable"] = false;

Refer demos/editing/default-edit.php and turn check by setting it.

$col = array();
$col["title"] = "Date";
$col["name"] = "invdate";
$col["width"] = "50";
$col["editable"] = false;
$col["editoptions"] = array("size"=>20); // with default display of textbox with size 20
$cols[] = $col;