Invoices are automatically generated and sent as long as there is an active subscription, even if you haven't started any devices in a while.
To stop receiving them, you need to cancel your subscription:
1. Log in to your Cloud account at https://cloud.geny.io/signin
2. Go to the Admin interface and to the Billing section
3. Cancel your active plan
If you do not plan to use Genymotion SaaS in the future, you can also request an account deletion with this form.