You can filter you indexes to the events you wish to remove in the standard Splunk search view (i.e. flashtimeline). For example:
<your filtering search> | delete
As this is "risky", the user will need to be assigned the "can_delete" role.
If it is in specific indexes, and you wish to remove that WHOLE index, you can use the CLI tool, "clean", to delete data from that Index. For example
./splunk clean eventdata ...
These methods should be used with extreme caution as the effects can not be reversed. I would recommend reviewing your filtering search carefully, and also reading the documenatation on this subject first... it should not be taken lightly.