To stop apps from installing on your iPhone/iPad, you can disable automatic downloads in Settings > App Store, or for stricter control (like preventing kids from installing anything), use Screen Time > Content & Privacy Restrictions to block app installations entirely by setting "Installing Apps" to "Don't Allow". You can also stop apps from other devices automatically installing by turning off "App Downloads" under the App Store settings. Reddit +3
Tap Screen Time, then tap Content & Privacy Restrictions. Turn on Content & Privacy Restrictions. Tap iTunes & App Store Purchases, then choose restrictions for installing or deleting apps and in-app purchases.
For Android:
If you're a parent in the family group, you can choose the purchase approval settings for family members whose accounts are managed with Family Link. On your Android phone or tablet, open the Play Store app . In the top left, tap Menu > Account > Family > Manage family members.
Block unmanaged Android app installation
Block apps, app downloads, websites, and purchases on iPhone
First, manually delete any unwanted third-party apps from your Home Screen by pressing and holding the app icon until a menu appears, then tapping Remove App > Delete App. Then, go to Settings > Screen Time > Content & Privacy Restrictions. Tap iTunes & App Store Purchase > Installing Apps > Don't Allow.
You can set up parental controls to restrict the content your child can download or purchase from Google Play.
You can restrict apps and games on an Android device by using parental controls, which lets you choose the highest content rating you want to allow for downloads or purchases.
How to Stop Unwanted Apps from Automatically Downloading on Android
Delete apps on your Android device
Prevent App Store purchases
Open the Google Play Store on your Android device. Tap the Menu icon (three horizontal lines) and select My apps & games. Navigate to the Installed tab and find the app you wish to delete. Tap the app and select Uninstall.
To disable app downloads on your child's Android smartphone, you can use the Safe Lagoon app to set download permissions and block certain apps, thus managing their screen time and receiving notifications for any new app they wish to download.
If you can't delete an app that's not made by Apple, check your parental control settings. Make sure Deleting Apps is set to Allow, then try deleting the app again.
After you turn on content and privacy restrictions, do the following:
Yes, you can permanently block an app on Android using a mobile device management (MDM) platform. Once an app is blacklisted and policy is enforced, users won't be able to download or use it—even after device restarts or factory resets (if restrictions are set correctly).
Block apps from being downloaded
You can use the Restrictions settings to prevent yourself (or somebody else, for example your child) from downloading new apps. Open Settings and go to Screen Time.
Block installation of new apps from Google Play
How to Disable Pre-installed Apps on Android