This article describes how an organizational unit admin (OU admin) can ensure a person is correctly associated with their OU's storage allocation.

If you have a person who is no longer with your department, then moving them to NOTOU is not enough to remove them from your storage allocation. That is because the way storage management is currently being handled is not by "Person in (or not in) OU", but by "Person in (or not in) Department mapped to OU". 

You must also ensure that the department associated with them within CAT (and for that matter, People Search and DAT) is not a department that maps to your OU. If the department of record for the individual maps to your OU, then you will be responsible for their storage. In these cases, you must work with HR to get the department information to properly reflect their current affiliation.