There a few ways to fix that problem:

• Change the lockdown mode to package blocking or activity supression
• Add to your lockdown a whitelist (allow only your app) or blacklist (add GooglePlayStore)
• Add the Google PlayStore to the blacklist by using the script command manualblacklist add com.android.vending
• Run your App as SingleAppMode (only when you use one app in your lockdown)
• Enroll your devices as Android Enterprise devices and deploy your app with the Managed Google Play Store
• In case you are already using the Managed Google Play Store .. make a new App Policy which contains
  only the apps you use in your lockdown

I recommend to enroll the devices to Android Enterprise because of the Managed Google Play Store. If they still get
to the PlayStore via Google Earth they wont be able to open or install other apps.

Hi Ricky,

You haven't mentioned much details about your device brand/model, enrollment mode, firmware version, apps included in your lockdown/kiosk, etc.   The best solution varies with all of the above.

While the application-run-control and lockdown policy changes mentioned by Ozan definitely help, you may consider also adding extra opition(s) in feature control profile payload to disallow Google account addition or application installation/unistallation.