This should automatically be managed by the Ironport for the logs.
If you go into each individual Ironport under System Administration -> Log Subscriptions. There should be a setting under each log on how many files it will maintain. Once that number is reached, the oldest file will be deleted on creation of a new log file.
There should be no need to manage the erasing of logs.
Christian Rahl
Customer Support Engineer
Cisco Web Content Security Appliance
Cisco Technical Assistance Center RTP