the delete command does not delete data from the index, the events will just no longer show as results but are still in the index. Also Splunk still knows what was indexed and therefor does not re-index the same data again .... unless you do some work.
The radical method is to clean the fishbucket index. That will remove the memory of every files, be warned it will re-index all data.
on an indexer splunk clean eventdata -index _fishbucket
on a forwarder by removing the folder $SPLUNK_HOME/var/lib/splunk/fishbucket
You could selectively forgot a single file from the fish bucket like this: