Prevent Gravity Forms from storing entries on a specific form.

gistfile1.php

<?php
add_action( 'gform_after_submission_5', 'tgm_io_remove_form_entry' );
/**
 * Prevents Gravity Form entries from being stored in the database
 * for a specific form. In this case, the form ID is 5. Change 5 in
 * the hook to target your specific form ID.
 * 
 * @global object $wpdb The WP database object.
 * @param array $entry  Array of entry data.
 */
function tgm_io_remove_form_entry( $entry ) {
    
    global $wpdb;

    // Prepare variables.
    $lead_id                = $entry['id'];
    $lead_table             = RGFormsModel::get_lead_table_name();
    $lead_notes_table       = RGFormsModel::get_lead_notes_table_name();
    $lead_detail_table      = RGFormsModel::get_lead_details_table_name();
    $lead_detail_long_table = RGFormsModel::get_lead_details_long_table_name();

    // Delete from lead detail long.
    $sql = $wpdb->prepare( "DELETE FROM $lead_detail_long_table WHERE lead_detail_id IN(SELECT id FROM $lead_detail_table WHERE lead_id = %d)", $lead_id );
    $wpdb->query( $sql );

    // Delete from lead details.
    $sql = $wpdb->prepare( "DELETE FROM $lead_detail_table WHERE lead_id = %d", $lead_id );
    $wpdb->query( $sql );

    // Delete from lead notes.
    $sql = $wpdb->prepare( "DELETE FROM $lead_notes_table WHERE lead_id = %d", $lead_id );
    $wpdb->query( $sql );

    // Delete from lead.
    $sql = $wpdb->prepare( "DELETE FROM $lead_table WHERE id = %d", $lead_id );
    $wpdb->query( $sql );
    
    // Finally, ensure everything is deleted (like stuff from Addons).
    GFAPI::delete_entry( $lead_id );

}