You can use the | delete command to make the data inaccessible to the users. This won't free up space on the indexers though.
http://docs.splunk.com/Documentation/Splunk/6.6.0/SearchReference/Delete
For this you need a user with can_delete role(if you already don't have then create a temporary user and assign this role) and then run the query to return all data and delete. index=tomcat earliest=0 | delete and delete the data.
To permanently remove event data from a single index, type:
$SPLUNK_HOME/bin/splunk clean eventdata -index <index_name>
Details are provided here - http://docs.splunk.com/Documentation/Splunk/6.6.1/Indexer/RemovedatafromSplunk
For freeing up space, remove any monitors sending data to this index, remove index configuration from indexes.conf, push bundle and delete the index directory from the indexers $SPLUNK_HOME/var/lib/splunk/tomcat .
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