Never fear because you need no software that users can get around. you need something like OpenDNS.com (they have and instructional video) where for free (if you use their service - it will make your surfing go fast because of DNS caching) you can block and web site you want.
Well if you watch the video and went to their router setup page and choose your router brand and it will walk you setting up their DNS in it. Then if create a free account you can go into Sign in/free account signup and then go to the "Settings tab" and you can add (near the bottom of the page) and add or block any site you deem necessary.
If you're experienced enough with routers and gateways, I highly recommend putting an Untangle box in "front" of your network. It does spam filtering, access control, website filtering, anti-virus, anti-malware, VPN and just about anything else you can think of.
I have a simple one set up that simply filters spam, but it's super-simple to set up website filtering as well.