Configure your Vm's to use host-only networking. Then configure the guest OS's to use a manually configured IP address without a gateway address in a range different from the host (your home network).
Let's say your mac uses 10.0.0.x network and your router (default gateway address) is on 10.0.0.128. Configure your guests to use an address in either 172.16.x.x. or 192.168.x.x. range.
I use this setup myself sometimes when I don't want my Vm's to access the internet. Works just fine for what you are describing.
In the example above the vm's won't be able to communicate with the host either. If you want guest communication but only disabble internet access use host-only and configure the guest OS's with an IP address in the same range as the host (ie, in the example from the 10.0.0.x range).