If you don't want any email going out, you can set the email driver to null or array.
Another driver option is log where all emails will be written to Flarum log and and not actually sent, useful for debugging but probably not useful for disabling email since null will do the same without unnecessary disk writes.
To disable just the email validation but not other kind of emails would require hooking into the Flarum PHP extension API. There's probably a way to intercept either the job that generates the email, or modify the mailer to dismiss that particular email. If you have a working SMTP server you could potentially configure a rule there to drop outgoing emails with a specific title.
