Block network with Network Link Conditioner?
Assuming you really want the answer how to block traffic rather than using the tool you have at hand, I'll suggest you use Little Snitch.

It is a firewall that works on interrupting the flow of data out from apps to the internet.

Perhaps someone will know how to bend the Network Link Conditioner to work as a firewall for the one app, but I'd start with one of the tools for that job if no one has a good recipe for the latter.