Are they spam submissions or are they mostly misdirected submisions? The following settings can help you reduce spam submissions and misdirected submissions.
Only registered user can add new link (yes)
Enable User Validation (yes)
Require visit category on submission(yes)
In captcha.php you can change the following
Code:
$key = GenerateRandomString(4, TRUE);
to
Code:
$key = GenerateRandomString(5, TRUE);
to confuse autosubmitters that rely on indexu only using 4 characters.
If you want to go up to 5 characters you need to change the following as well
Code:
// box color
$image = imagecreatetruecolor(81, 31);
$back = imagecolorallocate($image, intval(rand(224, 255)), intval(rand(224,255)), intval(rand(224, 255)));
imagefilledrectangle($image, 0, 0, 81, 31, $back);
to
Code:
// box color
$image = imagecreatetruecolor(90, 31);
$back = imagecolorallocate($image, intval(rand(224, 255)), intval(rand(224,255)), intval(rand(224, 255)));
imagefilledrectangle($image, 0, 0, 90, 31, $back);
Also remember, that some sites just don't have pagerank or they don't have pagerank yet. You can't base all submissions on that factor alone