[kh99]

I think what you want is in the file packages/vbforum/search/type/thread.php, so you could edit that file and comment out the line that adds that option.

You might be able to do it with a plugin by using hook template_register_var and code like:

if ($name == 'mod_options' AND is_array($value))
{
    global $vbphrase;
    unset($value[$vbphrase['merge_threads']]);
}

but I haven't tested that at all.