To unsubscribe from additional storage 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:
By clicking the button, you'll get to "Upgrade your storage" page:
Click "Unsubscribe" from the disk space you purchased earlier. Click "Confirm".
Next information will be about deactivated the subscription.