Locking Date and Time Preferences

Hi there - can anyone help me with this please?

I have the Netnanny filter installed with time restrictions - it's set so the net can't be accessed after 10pm.

Trouble is, after a restart on my 2007 iMac (OSX 10.5.8), the lock re-opens in Date and Time preferences, the time can be reset, and the filter software is rendered useless...

Can anyone suggest a way to stop the lock opening after a restart?

Thanks!

Whoever you need to use the computer needs to log-in to a standard account, not an admin account.

Basically, when you are entering your admin account passord on log-in, its unlocking preferences you have admin access to.