First you have to set the user notification type to 'handled in odoo'. Then you can create an automated action to mark certain notifications as "read". (as_read = True).

I applied my automated action on any notification where the message.subject contained "assigned" or "Delivery Order". "assigned" takes care of those notifications that someone assigned you an activity. (Why send me a notification when I see that I have a new activity? It's redundant.) "Delivery Order" is for shipment notifications. We didn't want all those notifications that an order was shipped. It actually still sends the message, but it doesn't show up in the "conversations bubble" at the top of the screen.

Odoo was becoming that friend who never stops talking, so you just start ignoring him. Then you miss something important because you weren't listening.

Hello Andreas,
You can disable those emails for each users like below:
go to users in settings menu,inside the user page you can see 'Notification' option under Preferences tab.Change it from 'handled by email' into 'handled in odoo'.I hope it will work.

Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari