Ok, I've tested it for a few days now and it seems to have no adverse side effects. Just create a firewall rule to WHITELIST outgoing traffic from nbminer.exe only when it's traffic to your pool's IP/port. That will successfully prevent it from phoning home.

The miner is sending encrypted traffic to srv.nebu.ltd once in a while, which I assume is their C&C server. Since I can't see what it's exfiltrating, I just block it in the hosts file. Would it have any adverse effect on the mining?