Provided the automated email has not been sent to the candidate, click the trashcan icon for the automation alert on the candidate's timeline. This will cancel the automated email.
Alternatively, you can move the candidate out of the stage where the automation will occur. An automation can only run if a candidate is in the relevant stage. The automation will be attempted again if the candidate is moved back into the stage. Automations will not be attempted again if they have been completed for the candidate already (i.e. the candidate will never receive duplicate automated emails).
Automations that are set to run "immediately" may not provide time to allow cancelling. We recommend setting a short delay for all automations in case you need to cancel.