Disabling sending of the confirm email

Are you wanting them even confirmed or are just purely trying to stop the emails?

I want purely trying to stop the welcome emails. The remaining is fine, although the user is active and confirmed

There's no way to stop it from any of the available triggers. The best I can suggest is to override the language strings and added IF substitution conditions to them to force them empty since CB won't send an empty email.

Maybe there was a problem of understanding. I will try to explain better what I want to do.

What I would like is what follows.
In the registration form there is a field (not mandatory). If this field is filled with a predetermined value (numeric or alphanumeric), then the confirmation email and email of successful registration should not be sent.
If the field is not filled or is filled with a different value than the predetermined one, then the standard emails are sent.

Basically, I would create two types of registrations regulated by this field.

You'll need to condition the emails themselves for that. There's no trigger to stop the emails. So for example you could use the below for the Pending/Welcome email body.

[cb:if cb_myfield=""]EMAIL_CONTENTS_HERE[/cb:if]

If the field has a value then no email body will be available which should result in the email being suppressed. You may need to do the same for the subject. Aside from that I don't know what to tell you as again there is no means to stop these emails via trigger.