//Change AWeber Error Message
add_filter( 'ninja_forms_aweber_subscribe_error_message', 'nf_change_aweber_error', 10, 3 );

function nf_change_aweber_error( $msg, $subscriber, $list_id ){
	if( strpos( $msg, "Email address blocked" ) ){
		$msg = "Sorry but you can't use that email address. Please try another.";
	}
	return $msg;
}