This plug-in marks events which are fully booked as red in the fullCalendar and removes the link to the event.

event-organiser-calendar-event-booked.php

<?php 
/*
 * Plugin Name: Event Organiser Calendar Booked Event Highlight
 * Version: 1.0
 * License:       GNU General Public License, v2 (or newer)
 * License URI:  http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 */

/**
 * This plug-in  marks events which are fully booked as red in the fullCalendar
 * and removes the link to the event.
 *
 * To use this place this file inside your plug-ins repository and then activate the plug-in
 * "Event Organiser Calendar Booked Event Highlight"
 *
**/

/**
 * Changes the event colour, text colour and removes the link (if the event is fully booked)
*/
function my_mark_event_as_booked( $event, $event_id, $occurrence_id ){
	
	//Check if the event has tickets
	if( eo_get_event_tickets( $event_id ) ){

		//Check if the event has any tickets remaining
		if( eventorganiser_pro_get_option( 'book_series' ) ){
			$remaining = eo_get_remaining_tickets_count( $event_id );
		}else{
			$remaining = eo_get_remaining_tickets_count( $event_id, $occurrence_id );
		}
		
		//If not change colour and url
		if( 0 == $remaining ){
			//Change event color to red 
			$event['color']     = '#ff0000';
			$event['textColor'] = '#000000';
			$event['url']       = false; //remove link (optional!)

			$event['title']      = $event['title'] . " (Fully booked)"; //change title (optional!)
			$event['description'] = "<strong>Fully booked.</strong> <br>" . $event['description']; //change tooltip content (optional!)
		}
	}
		
	return $event;
	
}
add_filter( 'eventorganiser_fullcalendar_event', 'my_mark_event_as_booked', 10, 3 );


/**
 * Because the calendar is cached, we need to clear it whenever a booking is confirmed.
 */
function my_clear_calendar_cache( $booking_id ){
	delete_transient( 'eo_full_calendar_public' );
	delete_transient( 'eo_full_calendar_public_priv' );
}
add_action( 'eventorganiser_confirmed_booking', 'my_clear_calendar_cache' );