You can use a module called ‘Dansguardian’ which can be configured and controlled via webmin using the associated plugin. Dansguarian will allow you to block access to specific websites only, or block specific LAN IP’s from accessing the Internet. It works quite well.
You’ll need to configure Dansguardian as your ‘proxy’ e.g port 8080, which firstly filters all network requests according to your policy, and then forwards valid requests to squid which runs on a different port e.g 8081.
Hope that helps.