As you cannot force users to update the app using Google play or AppStore build-in feature, you can use third party librairies that do exactly what you want. appgrades.io allows you to block any version of your app by displaying a view or popup (that you can customise on appgrades Dashboard) with a custom message to ask your users to update the app. You can even add an update button that sends the users to the appstore/Google play where they can update the app. No code needed. Just integrate the SDK in your app and block any version anytime.
The other solution would be coding what appgrades does by checking with your api the latest available version and block the app with a view when you are running a lower version.