Following command can be used to delete all the existing messages in kafka topic:
kafka-delete-records --bootstrap-server <kafka_server:port> --offset-json-file delete.json
The structure of the delete.json file should be following:
{ "partitions": [ { "topic": "foo", "partition": 1, "offset": -1 } ], "version": 1 }
where offset :-1 will delete all the records (This command has been tested with kafka 2.0.1