Deleting a user will remove the user and any data associated with the account—including uploaded creatives, purchased templates, and created campaigns. Once a user has been deleted, there is no way to reverse this action or restore the user’s data. BE CAREFUL DELETING USERS.
Deactivating a user restricts the user's access to the account (they wont be able to login any longer), but leaves all data associated with the deactivated account intact and accessible to the other users with relevant permissions. This is the best way to prevent someone from accessing your ads account or data on your account while preserving the work done by that user.