As you can't find them on the system, you can reply from Ezekia saying that you cannot find them and asking them for more details about their profile.
Navigate to the compliance screen and switch to the 'Right to be forgotten' tab (1). Enter the email address you have (2) and when no matches are found, click 'Reply to individual' (3). On the next page, enter the name and email details you have for the person. You will then see the email that will be sent - click confirm (4).
If they reply with valid details that can be found, you can use the ‘Right to be forgotten’ process to fully remove them from the system.