How to remove all documents in index

There is a way: query all documents, and use IndexBatch to delete these guys.

    public void DeleteAllDocuments()
    {
        // Get index
        SearchIndexClient indexClient = new SearchIndexClient(SearchServiceName, SearchServiceIndex, new SearchCredentials(SearchServiceQueryApiKey));

        // Query all
        DocumentSearchResult<Document> searchResult;
        try
        {
            searchResult = indexClient.Documents.Search<Document>(string.Empty);
        }
        catch (Exception ex)
        {
            throw new Exception("Error during AzureSearch");
        }

        List<string> azureDocsToDelete =
            searchResult
                .Results
                .Select(r => r.Document["id"].ToString())
                .ToList();

        // Delete all
        try
        {
            IndexBatch batch = IndexBatch.Delete("id", azureDocsToDelete);
            var result = indexClient.Documents.Index(batch);
        }
        catch (IndexBatchException ex)
        {
            throw new Exception($"Failed to delete documents: {string.Join(", ", ex.IndexingResults.Where(r => !r.Succeeded).Select(r => r.Key))}");
        }
    }

No, currently there's no way to delete all the documents from an index. As you suspected deleting and re-creating the index is the way to go. For really small indexes you could consider deleting documents individually but given that often apps have code for index creation already, delete/recreate is the quickest path.