How can I restrict apps from using WiFi?
There is no ios level switch for preventing WiFi or USB tethered  data connections on a per app basis. Disabling WiFi you don’t trust is the first step. 

The next practical method to do this is control the network with firewalls, proxies and DNS / routing to block whatever access patterns you need to control or monitor. 

Unless the specific app has a sign off / sign out/ disable feature or configuration option, deleting the app entirely is the last lever available if you cannot control the network.