I've got a VirtualBox machine set up and it runs fine. But I want to limit it's network access to only computers on my LAN (192.168.2.x). I do not want it to have any type of incoming or outgoing access to the internet at all.
Just remove the Default Gateway in it's IP settings. Or modify the operating systems hosts file.
If your really serious about blocking it though you should block it via a firewall.