i think this is what your looking for its called Broadband internet lock security
it can Disconnect your PC from your broadband Internet connection manually or automatically. Set session time limits. Further restrict access with password protection. the only problem is it is demo but you might be able to by the licence for the full version even give the demo a go,The link to the program can be found here
http://www.programurl.com/software/disconnect.htm and scroll down untill you find Broadband internet lock security