Restricting Web Access on A Network: Proxy Server?

I would like to severly restrict web access on a portion of my network. Basically I want all web traffic to redirect to the same site. This restriction will be for a wireless network (run from either a single Airport express or a single Airport Extreme (N) basestation). Clients will connect to the airport and be allowed access to this one site with any other URL directing them to this same site. What is the simplest way of doing this? Could I do it using DNS? I can setup a DNS server exclusively for airport clients and make the basestation supply that as the default DNS, but could that be easily bypassed by entering a DNS server manually? I had a quick look at parental control software, but this seems expensive and wasteful. Could I do it with a proxy server? Can I force clients to use it? I have seen this done with public pay-per-use wifi hotspots. Many thanks in advance, War