To unsubscribe from additional Disk space you would go to your Events page, click Billing in the drop-down menu:
Pushing "Billing" should take you right to the "Overview" page. You can find "Add space" written in red letters:
You will be redirected to the Subscription Purchase page:
Click the "Unsubscribe" button from the disk space you purchased earlier.
To confirm, click "Unsubscribe".
You will be redirected to the page "My Subscriptions", which will contain information about cancellation of subscription.
Also, you can keep the subscription by clicking on the button "Keep this subscription", or upgrade the disk space.