Security/privacy feature for spacedesk is not yet available.
This feature is already in our todo list but will not be available anytime soon.
We are currently very busy with higher prioritized features and bug fixes.
If at all, this feature will be part of the future PRO version.
If you want a private network between your primary and secondary machine, we can only suggest you use WiFi Direct/Hotspot or USB tethering (if supported by your secondary device).
These connection approach will establish a private network connection between your primary and secondary machine and will prevent others to discover your IP address unless they connect to your private network (with WiFi Direct/WiFi Hotspot you can also set a password).
For more info please check: