Hello, 

First of enable the Developer mode. 

Then move to  Settings -> Technical --> Discuss--> Subtypes
Here you can find all the action which related  to  default  actions. 

You can unchecked defaut in "Disucssion". 

Also, You can checkout this link for the Ref. https://www.odoo.com/forum/help-1/how-can-i-stop-emails-being-sent-automatically-to-followers-in-certain-conditions-when-so-confirmed-when-invoice-created-etc-142594

Thanks.