limiting internet usage via group policy

I am trying to place a restriction on 2 user accounts within a domain to prevent them accessing the internet for more than a couple of hours a day.
My initial thought was I should be able to do this via group policies.
However, I can find no policy which will allow me to create this rule, it seem to be either access or no access wich would be no good.
Is there a quick trick I can apply to enable this?
Any help greatly appreciated.
Kind regards, Damian.