Is this for 1.3.7 or from ES 2 version onwards

Deleting mappings
Finally, it is no longer possible to delete a type mapping (along with the documents of that type). Even after removing a mapping, information about the deleted fields continues to exist at the Lucene level, which can cause index corruption if fields of the same name are added later on. You can either leave mappings as they are or reindex your data into a new index.

ES 2.x