Go into /wpsc-theme/functions/wpsc-transaction_results_functions.php
Comment out the following
wp_mail( $email, __( 'Order Pending: Payment Required', 'wpsc' ), $message );
wp_mail( $email, __( 'Purchase Receipt', 'wpsc' ), $message );
wp_mail( get_option( 'purch_log_email' ), __( 'Purchase Report', 'wpsc' ), $report );