@marijn, open up the theme’s functions.php, remove the require PADD_FUNCT_PATH . 'prelude.php';,
and then remove the <?php padd_theme_credits(); ?>. Those themes are encrypted, I would do as suggested and find another theme.
Just for reference though, the encryption is in prelude.php inside the includes folder, there are plenty of online resources to decrypt them, check out this post: why-you-should-never-search-for-free-wordpress-themes-in-google-or-anywhere-else
Have fun
