When you are downloading updates from Mac App Store, you can pause the update of an app. But what if you want to cancel it altogether? Luckily, App Store offers that functionality as well.
Go to Updates in App Store and press and hold Option key. This will change Pause button into Cancel button for all apps that are currently updating. Click on Cancel button while holding Option key, and the update for that app will be cancelled.
However, if the app has already finished downloading and has started installing, you won’t be able to cancel the update. In this case just wait for the installation to finish and remove the app.