@Markus Männl , Based on my understanding, for the "Hidden network" in WiFi setting. if we set it as enable, it makes the device to connect to the network even when it is not broadcasting in SSID. That is to say, we can use this to let some specific users to access this hidden WiFi.
https://learn.microsoft.com/en-us/mem/intune/configuration/wi-fi-settings-ios#basic-profiles
In General, to hide a WiFi network, we need to disable the network's SSID by disabling SSID Broadcast option on the WiFi.
From your description, it seems you want to prevent some users to connect the open WiFI network. Currently I can only think that we can disable SSID Broadcast on the WiFI side and deploy the profile from Intune to the users who need this open WiFi.
Hope it can help.
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