Apple - Restrict an iOS app from using Wi-Fi internet

I'm sorry for it's not possible but you can disabled the app from running. These are the steps to follow:

Go to General,

Click on Restrictions,

Enter your device password,

Switch off the App you want to disconnect.

I use it to stop Facebook from consuming Data at the background.

FOR NEWER OS VERSION 12.4.6, YOU CAN FIND SIMILAR OPTION IN:

Go to Screen Time,

Click Content & Privacy Restrictions,

Turn on Content & Privacy Restrictions,

Then select what you want to restrict e.g Background App Activities.

It is possible to restrict WiFi or Cellular data for all the apps on your iPhone, but only for iPhones sold in China.

Chinese iPhones have an extra option called "Apps using WLAN & Cellular" in Settings > Wi-Fi.

In that section you can control apps from accessing data on Wi-FI or Cellular. If you don't want that app to access data, there is "Off" option and the app can't access data on cellular or Wi-Fi.

On stock ( or as some people call it vanilla ) iOS it is not quite possible.

However, if your device is jailbroken, then you can use some tweaks to make it possible. Firewall iP7 allows you to monitor all applications & decide whether to allow or deny any outgoing connection.