I know one can delete all documents from a certain type via deleteByQuery.
Example:
curl -XDELETE \'http://localhost:9200/twitter/tweet/_query\' -d \
Torsten Engelbrecht's comment in John Petrones answer expanded:
curl -XDELETE 'http://localhost:9200/twitter/tweet/_query' -d '{ "query": { "match_all": {} } }'
(I did not want to edit John's reply, since it got upvotes and is set as answer, and I might have introduced an error)