In the event that the customer's account becomes "Inactive" because of a failed payment due to credit card issues or simply because their trial period ended and they are not interested in subscribing. you would want to prevent their site visitors to have access on their site but not totally delete or deactivate the subsite.
WP Ultimo has a feature that will allow you to block frontend access on the site should their account becomes inactive.
You can easily set this up by going to WP Ultimo > Settings > Network Settings. Then under Network Options you can find a checkbox for "Block Frontend Access".
Under this option you will see a field that will allow you to set a grace period if you wish to delay or wait for certain number of day(s) before the site frontend access gets blocked.
Once the customer's account is "Inactive" and the grace period has already past, site visitors will see a message saying that the site is not available.