Set the last available date in the JetBooking calendar. Change the '2023-12-31' to the needed date

end-date-jetbooking.php

<?php

add_action( 'wp_head', function () { ?>
<script>

    jQuery( document ).on( 'jet-booking/init', () => {

        window.jetBookingState.filters.add( "jet-booking/input/config", modifyBookingConfig );
        window.jetBookingState.filters.add( "jet-booking/calendar/config", modifyBookingConfig );
        
        function modifyBookingConfig( config ) {
           
            config.endDate = '2023-12-31';

            return config;
            
        }

    } );
    
</script>
<?php } );