Block Web portals through Group Policy

I did think this might be possible in a crude way by adding the sites to trusted sites and then not allowing IE to access anything else but on reflection this is not a good idea as the 'trusted sites' would have more provilages on your machine than they really should have any in case. You are alow going to get all sorts of issues with links an referals that would make it a nightmare to keep on top of.

If you really want to do this bite the bullet and spend a little of your hard-earned on a decent proxy sever such as Microsoft ISA server,

The sites I am allowing should be trusted site anyway so I am not worried about the extra privillages.  I have not found a way to only allow the trusted sites and block everything else.  In general I agree with you, but in this situation I think it will be OK from a security standpoint.

I figured out a way to do this.

It is a proxy solution where you send everything to a false proxy (I just choose an unused IP address, or someone you really don't like).  Then put all the websites you want to work in the do not use proxy for these.

In Group Policy :
User Configuration / windows settings / Internet Explorer Maintaince / Connection / Proxy settings

I set port http and https to use the proxy

in the same area, I configured  the exceptions to be the web sites I want people to get to.
I was able to use wildcards so I could allow an entire domain.

This will only work if you have a limited number of web pages you want to allow.  This worked great for us because my users only need about 12 site to be able to do their jobs.