How can I disable email alerts for specific users?

There isn't a way to change this behavior by default. However, each of your users can change this setting from within their accounts in the Cloud Manager. They can click on their username in the top-right section of the screen, click on "My Profile," click on the Settings tab, and toggle off "email alerts for account activity." This will prevent emails from being delivered for that user for any account activity, including tickets and billing.