One free option is to use OpenDNS. A personal account is free, and has filtering capabilities. You set your router to use the OpenDNS domain name servers, and every device connected to that network is affected.
Of course, this isn't perfect. Someone knowledgeable could use a VPN to "tunnel" outside your network and access anything they like. But no such blocking technology is perfect.
You could go into your router security settings and block the website from there. You will need to change the router login so that it has a good password. That way you (and only you) can make changes.
A real persistent and smart user will probably eventually figure out a way around this, but for most users this will be fine.