Try using OpenDNS.(
OpenDNS | Providing A Safer And Faster Internet)
First change your DNS servers to 208.67.222.222
and alternate DNS to 208.67.220.220.
Then sign up for an account on the OpenDNS site and first select
networks add your network then go to your
dashboard select
content filtering and click on the
customize link ,tick everything except
webmail.Then only webmail sites can be opened.
Your done with it (don't tell your OpenDNS password to any of your guests or clients)
It is
free and you don't have to install any software also if you are having a static IP if not there is a small software called IP address updater on their site itself .Install it and your done.