In v16 if you create a plan for your subscriptions Odoo will automatically create, post and send by mail your invoice. 

Even if you leave the field "invoice mail" empty you will still send the email of your invoice.

You have 2 ways of solving this:

1)

• Create an external email server => add a wrong smtp server which will prevent you from sending email (you can check by clicking on the "Test connection" button.

You need to have also another outgoing email server that is working which has a higher priority in order to be used for all other emails send from Odoo

• Create a new email template

Create a fake email template that you will link to the subscription plan and to your blocking outgoing email server

As you linked your plan to your email template that cannot be sent as linked to a blocking email server then your invoice will be created and posted but not sent

They will be in the email menu but will never be sent

2)

• Go in debug and in the external identifier menu
• Search for this external identifier: sale_subscription.mail_template_subscription_invoice

This external identifier is the one called by the action send the email of the invoice automatically. 

• Change the name of the external identifier

As the name is now changed, no email will be created as the action called is now not found anymore (because we changed the name)