On an Apple Airport you can setup Guest network access, first, are you using this because it is very, very important. If not, turn it on. However, if you have an old Airport, the Guest network account feature is not available. The Guest network is only available on Airport Extreme and TimeCapsule that was introduced earlier this year (2009) and newer. Your firmware version does not tell you if your Airport can use Guest networking, you have to know the version of your Airport or T.C.
I will assume that you have an Airport or T.C. that does have Guest networking.
If Guest networking is turned off, you need to enter the Airport Utility and go to the Airport -> Wireless tabs (unfortunately I don't have a Airport to do this, so as I recall, you will see a button, or field where you can go to enter the Guest network info). When you turn on Guest networking, and give it a name (other than your current SSID (this is the wireless network name, but SSID is shorter), or change you current SSID to something else, and give you Guest network the old SSID you were just using).
Now with a Guest network in the same window you can "turn off file sharing between guests". Now, if they are Windows PCs, I suspect this may not turn of P2P for them. This is a bit more tricky. I would also strongly urge you, regardless of this network now being isolated from your other personal network, you will want to put a password on it, and I'd suggest WPA2 Personal, or at least WPA Personal!
You need to find out the TCP/IP "ports" that are used. You will also need to setup DHCP with NAT or what Apple likes to call "Share a Public IP" under the Internet -> Internet Connection tabs. You'll need to setup DHCP and NAT as you like. Set it up so that you have some "unused" IPs in your DHCP subnet range (i.e. in your fourth octet use only 1-100, this means that 101-254 are not "unsed" by DHCP if you use a 255.255.255.0 subnet). At least one of these unused IPs will be used in the next step.
When done there, go to the Advanced -> Port Mapping tabs. This is where you'll set the P2P ports up to be forwarded to one or more of the "unused" IPs. The reason I suggest keeping it in the same subnet is so that your Airport (or T.C.) will never send this traffic outside itself. It thinks the IP is somewhere connected to it. And since it's not your other "guests" will never leave your Airport with their P2P traffic.
Man, this is more complex than I realized, but it will work. It may be easier to tell everyone to buy an Apple and simply use the, "turn off file sharing between guests," feature. Ok, ok, it's just a suggestion.