How to block website on internet explorer?

As long as there is a route to the internet, it is relatively easy to get around web site blocking (i.e. - with 3rd party browsers, proxies run off a pen drive, etc.). If the intranet site is on the same network as the clients - that is you don't need to go through a router, then you can remove the Default Gateway IP address from the DHCP server or on the Properties sheet for the network adapter.

If this isn't feasible, you might route the client computers through a proxy server that allows access to only your Intranet site.

I'm going to assume you are wanting to block specific users or rolls rather than specific machines from accessing the Internet. If this is true, you should be able to do this via a group policy.

Going the proxy route - the better proxies can point toward a white list for the user and restrict that way.

If on the other hand, it is specific machines you want to control regardless of who is on them, then access restrictions at the router level can work. Again, this is normally done through a white list attached to a specific IP address or hardware device ID. Obviously this won't work with dynamic IP addresses.