There are a couple things to check on here.
1. UIM hubs will broadcast to find other hubs;
2. UIM Robots will broadcast to find secondary hubs in certain circumstances.
For hubs, you can disable this functionality by changing "Broadcast On" in the Settings/General tab of the hub GUI:
Or you can add/change the following in hub.cfg, in the main <hub> section:
broadcast_on = no
For robots, whether they broadcast to find a secondary hub is controlled by the checkbox "Search the subnet for a secondary hub..." in the controller config.
The raw configure equivalent of this is:
temporary_hub_broadcast = no
To search the subnet, robots broadcast a "getnearesthub" command so if you disable this checkbox or add the above key it should prevent this behavior.