To stop a follow-up email being sent, locate the "Automated Queue" section of your dashboard and select "View Queue".
This page shows the guests with pending automations.
Locate the guest you wish to remove and select "Delete" beside their name. This does not delete the record, it only deletes their email automation.
If you cannot find the guest in this queue, this may be because the automation has already sent based on the delay that was set.