I looked into this myself recently. I finally decided to use the access restrictions on my router rather than blocking logins on the workstation, since the real issue was limiting internet access, and there was no easy solution to setting up time of day restrictions on a stand-alone Windows Machine.
There was one commercial product that could do this. Google "parental control" software and you should find some options. I also found a free application designed to run an internet cafe that let you set up time of day restrictions, length of session restrictions, remote logouts, restricted websites, etc. It's called Dynasoft SurfShop and it's at
http://www.dynasoft.net. Looks like it takes some setting up, but should do what you need.
