How to prevent email creation when mail send to the address added include a certain subject

Hi Renaud,
 
When it comes to the Zendesk system behavior, all forwarded emails will automatically be converted to a ticket. It can also be a suspended ticket in some occasion. As it turns out, there's no feature available to restrict ticket creation. 
 
The best option here is to create filters/rules in your actual email settings, similar to Outlook rules, to restrict forwarding of emails with such conditions. If those emails will not be forwarded to Zendesk, it will not create a ticket at all.