How can i block internet access for a userin a pc

You can set up a fake proxy server for each user account. Of course, if they know how to disable that they will. You can prevent them from changing their proxy setting, but I'm not sure how in win 7. This works for vista so it may work for 7 also.

http://mintywhite.com/vista/vsecurity/disable-internet-access-in-windows-vistaxp/