An example of how to display a message and remove the ticket selector if the user has already registered onto the event before. The WP User Integration add-on is REQUIRED for this to work.

example.php

<?php //Please do not include the opening PHP tag if you already have one.

add_action( 
  'AHEE_event_details_before_the_content',
  'my_add_content_event_if_logged_in_registered', 
  11
);

function my_add_content_event_if_logged_in_registered( $post ) {
    if ( is_user_logged_in() ) {
        $user = wp_get_current_user();
        if ( ! $user instanceof WP_User ) {
            return;
        }
        //is there an attached EE_Attendee?
        $att_id = get_user_option( 'EE_Attendee_ID', $user->ID );
        if ( empty( $att_id ) ) {
            return; //bail, no attached attendee_id.
        }
        //grab contact
        $contact = EEM_Attendee::instance()->get_one_by_ID( $att_id );
        //if no contact then bail
        if ( ! $contact instanceof EE_Attendee ) {
            return;
        }
        // does the user have a registration for this event
        $registration = $contact->get_most_recent_registration_for_event($post->ID);

        if( $registration instanceof EE_Registration ) {
            echo '<p style="color:red">You have already registered for this event, and your current registration status is ' . $registration->pretty_status() . '. If you need to request any changes to your registration, please contact the course administrator.</p>';            
            if( is_single() ) {
                remove_filter(
                    'the_content',
                    array('EED_Event_Single', 'event_tickets'),
                    EED_Event_Single::EVENT_TICKETS_PRIORITY,
                    1
                );
            } elseif ( is_archive('espresso_events') ) {
                remove_filter(
                    'the_content',
                    array('EED_Events_Archive', 'event_tickets'),
                    EED_Events_Archive::EVENT_TICKETS_PRIORITY
                );
            }       
        }
    }
}