How to prevent Windows 7 from reading the BIOS time?

Rather than preventing it reading the BIOS time, the easiest option would be to set a BIOS password and not disclose it to anyone who might make any changes. While there are ways around that, my guess would be that your average family member isn't going to know how to do that.

Your operating system isn't running when the computer is turned off, so Windows has to read the hardware clock in order to get an accurate time. Otherwise, the Windows clock would "stop" every time you turned off your computer. Think of it with a metaphor: You can stand there counting "one-Mississippi two-Mississippi" etc to keep track of time, but if you fall asleep then when you wake up you'll need to look at a clock in order to know what time it is then, otherwise you'd just have to pick up from where you were before and you'd be off by the amount of time you were asleep.

Instead, you need to set a BIOS password to prevent people tampering with the BIOS time. Set an "Administration" password and that password will be necessary in order to change BIOS settings. The BIOS password can be reset by means of physically moving a jumper on the motherboard, this is one of the primary reasons that computer cases come with lock loops. If you really want to secure the machine, then you'll need to buy an inexpensive padlock and throw it through that loop.