OK, here is where you went wrong, rather the connecting your old Wrt300N modem to your current router, which is what you don't want and that's one of the reasons why you can still ping your main computer. Have both the modem and the router physically disconnected from each other. Once you have done that, connect your server that is running the virtual machines to your old Wrt300N modem, that's it. DO NOT connect your old Wrt300N modem to your router, because if you do you are not isolating your server from your main computer, you are simply connecting the two together and you don't want that.
With the server connected to the modem only, it should now be completely isolated from your main computer. Your main computer should not be able to ping the server or vice and versa, because its connected to the router and not to the modem, correct?