You can delete the Look In history via the 'Clear Search History' menu option:
Which has options for clearing various drop down lists:
More information
The Look In information is stored in the history.xml file usually located here:
C:\Users\<user>\AppData\Roaming\Mythicsoft\AgentRansack\config\history.xml
If you delete it you delete all search history. You can remove individual elements but they are base64 encoded so can be difficult to decipher.
