If you don't actually have an existing internet connection, set up an ad-hoc wireless connection between the machines or even better a stand-alone access point to serve a wireless network between them.
If you have a wireless router (like it sounds from your post) just leave the WAN (internet) port unplugged.
If you have a working internet connection but simply want to disallow a particular set of machines access to the internet, set them up with static IP addresses that are not included in your DHCP range and a blank default gateway setting. This is not 100% perfect as any savvy user could go and add the default gateway setting themselves and get internet (if they have sufficient rights on the computer), so you can further improve this if you configure firewall rules in the router to block access to the internet from the IP addresses of those computers.
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