The Airport Extreme doesn't seem to have an IP access-control list (ACL) to block outbound access to a particular IP address. Is there any way to add a null route or a blackhole for a specific IP so that it cannot be accessed from the local network? For instance, an iPad connected to WiFi would not be able to connect to the IP.
You can set the Airport to get DNS from a service like OpenDNS, and then set a blacklist at OpenDNS to block the IP you don't want the iPad to connect to. This would affect every device that connects to your Airport, not just the iPad though.
It's a solution, but not the one you were looking for. The forums at Macworld and Macrumors have a lot of people commenting about how you can't blacklist directly with an Airport Extreme, and this seems to be the solution that works for them. From one such post...
So, someone who is tech savvy could still manually set the iPad to use a different DNS server to bypass the OpenDNS option.