Just use the following secimg.php instead of the original one to get a captcha only with numbers.

secimg.php

<?PHP
session_start();
 
$secCode = rand(100000,999999);
$_SESSION['antispam_'.$_GET['formid']] = md5($secCode);
 
// get from: http://www.01-scripts.de/01scripts/01pics/sec.jpg
$im = imagecreatefromjpeg("sec.jpg");
 
// get from: http://www.01-scripts.de/01scripts/01pics/verdanab.ttf
$font = "verdanab.ttf";
$fontSize = 12;
$fontColor = imagecolorallocate($im, 70, 70, 70);
 
imagettftext($im, $fontSize, 10, 5, 25, $fontColor, $font, $secCode);
header("Content-Type: image/jpeg");
imagejpeg($im,NULL,100);
?>

Addon for https://gist.github.com/01-Scripts/535068