They likely set up their notification email as a mailer. To stop this, you'll need them to sign in, click the Events tab on the left. Click the filter they set up the alert for (example):
Then click the alert dropdown, select Not Subscribed, and click Update.
They can also Delete the filter if you prefer, or change their notification email in the Account Settings section. Click their username in the top right and then click Account Settings. Next, click the Edit button in the user section. Then change the notification email from whatever mailer they used to their own email address.
Hope that helps!
-Matt
