If it's a very sensitive issue, you might try to export the events from the whole index (or probably you could try exporting raw data from a single bucket with help from Splunk Professional Services), delete index files from server's disk, modify the exported events "offline" and ingest them again. But it's a very cumbersome process.
In general, once the data is indexed, it's immutable and stays in the index until it expires or is being pushed out because of index size limit. Even the delete operation does not really delete data from index files, it just marks the events as unsearchable.
