In ArcGIS Server 10.1 SP1, the only option you have for cleaning up log files is to clean the entire log. We have been importing specific log files into an Oracle table for Spotfire analysis. It would be great to be able to clear log files based on a query or message type (i.e. Clear all End MapExport Image logs). That way we can import the information we want to further analysis and leaving things like Warnings or Error messages.