hi ,
Please check the source of inventory of the users you are referring to unknown and those might be from the inventory sources like Citrix /office 365 integrations and even if you delete the users sometimes they may get created overnight for the next sync with the inventory source.
Update the user records with all the necessary fields as inventory sources may use email/sam account id/name for checking the user available else they will create a user named unknown.
You can delete the users directly by selecting them and delete if they are manually created and have to do housekeeping on the sources like AD/HR systems first so that they have all the fields populated complete user details and this will help the system not to create unknown users.
Regards,
Winvarma.
