Is there any setting available in hub Probe that lets you to force the hub to not accept new robot connections?
Currently not that I am aware of.
Only way I know of is to use a local firewall (windows/iptables/firewalld/etc.) and set it to exclude all traffic to your hub on the ports it's configured to listen on except for the robots you want connected.
Garin's point is good idea. If traffic is blocked, robot registration to hub will fail.