Example of how to hook into SPCO's attendee_information reg step and do some additional processing which can prevent the submission BEFORE spco has processed the data.

spco_hook_example.php

<?php

function attendee_information__process_reg_step($process_reg_step, $reg_step, $spco) {
    // Set the ID of the question you want to check the value of here.
    $question_id_to_check = 17;
    // Pull the transaction from checkout.
    $transaction = $spco->checkout->transaction;
    // Sanity check to confirm we have a
    if($transaction instanceof EE_Transaction) {
        // We need to process the primary registrants reg questions, to do that we need
        // the reg_url_link for the primary registrant.
        $primary_reg_link = $transaction->primary_registration()->reg_url_link();
        // No process the data from the reg_step
        $valid_data = $reg_step->valid_data();
        // Pull the primary registrant's answers using the reg_url_link we pulled earlier.
        $primary_reg_question_groups = $valid_data[ $primary_reg_link ];
        // Loop over each question group assigned to the primary registrant and pulls it's questions.
        foreach($primary_reg_question_groups as $group => $questions) {
            // Within each group process the questions to find one with our matching ID.
            if(!empty($questions[$question_id_to_check])){
                // Do additional processing to confirm value here!
                // return any falsey value to stop the processing, 
                // any truthy value will allow processing.
                $our_question_value = $questions[$question_id_to_check];
                if($our_question_value == 'stop') {
                    $process_reg_step = false;
                    break;
                }
            }
        }

        // Halt processing, add an error.
        if( !$process_reg_step ) {
            $error = 'SPCO has been stopped before registrations have been processed';
            EE_Error::add_error(esc_html__($error, 'event_espresso'), __FILE__, __FUNCTION__, __LINE__ );
        }
    }
    // Always return ptocess
    return $process_reg_step;
}
add_filter('AHEE__Single_Page_Checkout__process_reg_step__attendee_information__process_reg_step','attendee_information__process_reg_step', 10, 3);

function process_attendee_information__end($reg_step, $valid_data) {
      $reg_step->checkout->redirect = true;
      $reg_step->checkout->redirect_url = 'https://google.com';
      $reg_step->checkout->json_response->set_redirect_url($reg_step->checkout->redirect_url);
      return;
}
add_action('AHEE__EE_Single_Page_Checkout__process_attendee_information__end','process_attendee_information__end', 10, 2);

Note: $question_id_to_check is supposed to be a string as the $questions array is a type (string) => type (string) associative array.