Restricting/blocking Internet access from a system on the LAN (in a VM)

I'm a bit of a techy but not where networks are concerned, so any help would be appreciated as I've searched in vain for an answer, both here and the Net in general.

I have a Linux system running in a VM (VirtualBox) and, for testing purposes, I'd like to block access to the Internet but not to the internal network, from that VM. I have an NF4V router. I've tried various firewall and parental control settings but nothing seems to work. The NF4V manual really just lists the actual interface screens with very little extra information, and their FAQs don't cover what I'm trying to do.

Any ideas?