How to empty the log file
This article describes how to empty the FDSE search log.
If you are using file-based logging -- which is the default and most common format -- then you must FTP to your web server, change directory to the "searchdata" folder, and delete the "search.log.txt" file. This will clear the log. The log file cannot be cleared from the web interface.
If you are using SQL-based logging, then you can manually run the "DELETE FROM logs" command (customized for the named of your logging table). Or, under "Admin Page" => "Data Storage" => "Check Database Configuration", you may rebuild the logging table, which will delete all data.
"How to empty the log file" http://www.xav.com/scripts/search/help/1082.html