Restrict Internet access to a particular app only for a specific time period on an unrooted device
This doesn't apply to Wi-Fi, but the Android system has a per-app setting "restrict background data" that will completely stop the app from using mobile data except when the app is running in the foreground (i.e. it's the active app).

To access this setting, go to System Settings > Data Usage, then scroll down to locate the app and tap it. Then scroll down to find a checkbox for the setting.