Here are the steps to unpublish the Android app:
Login to Google developer console. (https://play.google.com/apps/publish)
Select the app.
Go to store presence on the Left panel.
Select Pricing & Distribution.
5. Click on the Unpublish button in the App Availability section.
6. Submit the update.
Here are the steps to unpublish the iOS app :
Log in to iTunes Connect. (http://itunesconnect.apple.com)
Select "My Apps" and select your app.
Go to Pricing & Availability on the left panel.
4. Select the "Remove from sale" option from the Availability section.
Note: Google and Apple developer platforms don't allow us to delete the app, we can only unpublish them and make them unavailable for download.