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 App Availability section.
6. Submit the update.
Here are the steps to unpublish iOS app :
Login to iTunes Connect. (http://itunesconnect.apple.com)
Select My Apps ans select your app.
Go to Pricing & Availability on left panel.
4. Select "Remove from sale" option from Availability section.
Note : Google and Apple developer platforms doesn't allow to delete the app, we can only unpublish them and make unavailable for download.