There doesn’t seem to be:
https://github.com/discourse/discourse/blob/8fb2e2a3ecc99e64ef4a1b127990e5b454cb3dbe/app/jobs/scheduled/activation_reminder_emails.rb#L11
The email will get sent to an unactivated user once, 2 days after they signed up on the site.
The purge unactivated users grace period days setting could be used to prevent it, but I think you’d have to set it to 1 day.
Realistically, if a user doesn’t activate their account shortly after signing up, does the activation reminder work to get them to complete the signup process?