While recently setting up user accounts with parental controls, I decided to test how well various websites are blocked. I was using the Guest Account, which as far as I know, cannot be deleted. When checking the logs after my nasty little trip through the internet, I discovered that the logs cannot be deleted through the Parental Controls menu. I found a way to do this using Terminal, but also found a way to do this successfully using Finder. You will only be able to do this if you have administrator privileges.
The logs are stored in: /Library » Application Support » Apple » ParentalControls » Users » username. Normally, you will be allowed to see the /Users folder, but you will not have access to it. Accessing the file and deleting the log is straightforward:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081212163548700