ESXi 5 host-only test environment
This would be accomplished by setting up a virtual switch (vSwitch) that does not have any physical uplinks to the rest of your network. Set the guest VMs to use this newly-created switch as their default. That will result in the desired isolation.
If by "talk" you mean the network access is only within the host itself. Simply setup a vSwitch that does not have physical NIC attached to it. Then on the virtual machine(s) set the NIC to be connected to that newly created vSwitch network label.