You'll likely have to modify the user object by reference in a code action before they're registered to do that. Something like the below might work.

Global
Triggers: onBeforeUserRegistration
Type: Code
User: Automatic
Access: Everybody
Action
Method: PHP
Code:Parameters
Reference Variables: Variable 1

Add whatever conditions necessary under the conditions tab, but I recommend testing without conditions first to see if it even works.

Thanks krileon, the code works. The email to confirm registration is not sent. Send though the confirmed registration email. I also want to block this email. How would I change the code?

The confirmation email does or does not send? Sorry, confused by this as you said it's not sent, but is sent?

I also want to better understand the management of variables. Where can I see a manual or tutorial on this subject?

Documentation on trigger variables can be found below.

www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers

I would also like to know how to set the "Return" parameter, I have left "echo"

There is no return during that trigger unless an error occurs. Return in this case should be set to Silent.

The usage provided causes the user to be automatically confirmed. This results in the Welcome email (e.g. YOUR_REGISTRATION_IS_APPROVED_SUBJECT) being sent as their registration is completed since no admin approval is enabled. Are you wanting them even confirmed or are just purely trying to stop the emails?

There's no means to stop the emails through any of the available triggers. The emails however will not send if the subject and message is empty so you maybe able to use IF conditions within a language override of the language strings for the emails to result in empty subject and body based off various conditions.