Add following code in your theme’s (child theme’s if you are using) functions.php

add_filter( 'fep_filter_tokeninput_localize', function( $args ){
	if ( isset( $_GET['fepaction'] ) && 'newmessage' == $_GET['fepaction'] && ! empty( $args['prePopulate'] ) && is_array( $args['prePopulate'] ) ) {
		$args['tokenLimit'] = count( $args['prePopulate'] );
		foreach ( $args['prePopulate'] as &$value ) {
			$value['readonly'] = true;
		}
	}
	return $args;
});