Disabling browsing issue help needed

Hi folks!
I have several computers that have internet access. I would like to have internet access on the computer, but I would like to have browsing disabled.
This needs to be disabled in Internet Explorer and Mozilla Firefox.
I have entered a fake proxy server in both browsers and that worked perfectly until the users figured it out and were able to remove the fake proxy settings.
How can people access internet in a computer just to receive emails, but disable the browsing so that they won't be able to browse the internet?
The solution should be one, that requests administrators permission to change.
Thanks in advance.
