This guide will walk you through how to remove your app from sale and making it unavailable for public download.
Apple App Store
1. Login to https://appstoreconnect.apple.com
2. Click on My Apps
3. Select the desired app you'd like to remove from the App Store
4. Select Pricing and Availability
5. Click on Remove from Sale
Android
1. Login to https://play.google.com/console/
2. Click on the desired app you'd like to remove
3. On the left menu, find Advanced settings under Release 
4. Select Unpublished and save.