There is no option to disable just that.
In the email tab of the user preferences, there is an option:
Email me when I am quoted, replied to, my @username is mentioned, or when there is new activity in my watched categories, tags or topics.
Users can set that to “never,” “only when away,” or “always.” You can configure the default for that preference with the default_email_level site setting. When you change that, you can choose to also change it for existing users. But it is not only about watched categories; it is also about @mentions and replies to their posts.