Truncate Table functionality is not available in DynamoDB, So kindly consider deleting the table & creating again,
Reason : DynamoDB Charges you based on ReadCapacityUnits & WriteCapacityUnits which you have used. If you delete all items using BatchWriteItem function, it will use WriteCapacityUnits. So, to save these WriteCapacityUnits for deleting items, It will be better if you truncate the table & recreate it agian.
Steps to Delete & Create DynamoDB Tables as follows :
Delete Table via AWS CLI :
aws dynamodb delete-table --table-name *tableName*
Delete Table via AmazonDynamoDB API :
Sample Request
POST / HTTP/1.1
Host: dynamodb.<region>.<domain>;
Accept-Encoding: identity
Content-Length: <PayloadSizeBytes>     
User-Agent: <UserAgentString>
Content-Type: application/x-amz-json-1.0
Authorization: AWS4-HMAC-SHA256 Credential=<Credential>, SignedHeaders=<Headers>, Signature=<Signature>
X-Amz-Date: <Date> 
X-Amz-Target: DynamoDB_20120810.DeleteTable 
{
    "TableName": "Reply"
}
Creating DynamoDB Table via AmazonDynamoDB API :
POST / HTTP/1.1
Host: dynamodb.<region>.<domain>;
Accept-Encoding: identity
Content-Length: <PayloadSizeBytes>     
User-Agent: <UserAgentString>
Content-Type: application/x-amz-json-1.0
Authorization: AWS4-HMAC-SHA256 Credential=<Credential>, SignedHeaders=<Headers>, Signature=<Signature>
X-Amz-Date: <Date> 
X-Amz-Target: DynamoDB_20120810.CreateTable 
{
    "AttributeDefinitions": [
        {
            "AttributeName": "ForumName",
            "AttributeType": "S"
        },
        {
            "AttributeName": "Subject",
            "AttributeType": "S"
        },
        {
            "AttributeName": "LastPostDateTime",
            "AttributeType": "S"
        }
    ],
    "TableName": "Thread",
    "KeySchema": [
        {
            "AttributeName": "ForumName",
            "KeyType": "HASH"
        },
        {
            "AttributeName": "Subject",
            "KeyType": "RANGE"
        }
    ],
    "LocalSecondaryIndexes": [
        {
            "IndexName": "LastPostIndex",
            "KeySchema": [
                {
                    "AttributeName": "ForumName",
                    "KeyType": "HASH"
                },
                {
                    "AttributeName": "LastPostDateTime",
                    "KeyType": "RANGE"
                }
            ],
            "Projection": {
                "ProjectionType": "KEYS_ONLY"
            }
        }
    ],
    "ProvisionedThroughput": {
        "ReadCapacityUnits": 5,
        "WriteCapacityUnits": 5
    }
}
Summary : Delete the table & Create it again would be the best solution.