Click the cog wheel in the top right corner of the screen to open up settings, then click settings in the drop-down menu!
Click the Manage Apps tab inside the settings pop-up window.
The Manage Apps tab will list out the apps that have been connected to your Google Drive. When you find an app you'd like to disconnect or remove its data, click the Options drop-down menu beside it.
While an app might not be connected anymore, some do show that their data is still being stored on your Google Drive. Choose to either disconnect or remove hidden app data from the menu and continue on to the next step!
Another pop-up will appear asking you to confirm if you'd like to delete the data. Hit delete to finish!