Stop Google Play Services Auto Updates

Google Play Services updates itself to the latest version automatically. To keep the downgraded version permanently necessary steps needs to be takes.

01. Go into your phone's Settings and click on 'Apps' or 'Installed Apps'.

02. Click on 'Google Play Services'.

03. Click on 'Mobile Data' or 'Data Usage'.

04. Disable any background data permissions the app has. (Some devices will have different names for the option. Just untoggle all the Data permissions).

05. If you need to downgrade your Google Play Services do it now. After downgrading your Google Play Services again go to Settings > Apps > Google Play Service and Click on Permissions.

06. Then untoggle Storage.

07. Go back to the Google Play Services app info menu and tap Storage.

08. Press the Clear Cache button.

09. Go back to your Settings > Apps > All Apps click the Google Play Store (not the services) and press the Disable button.

10. Go back to Settings > Apps and click on the 3 vertical dots ⋮ and select Show System Apps.

11. On that same screen click on the app Download Manager or Downloads and click Disable.

12. Restart your phone. 

That's it! Google Play Services shouldn't update anymore.