Very often these users are being (mis)used without people knowing it. You can start analyzing the users usage via Sm19/sm20 and/or the STAD or via specific software tools.
Before deleting the users, you can start by changing the passwords for these users. Very often you will see that the users are used for background jobs or similar (this shouldn't be the case, off course).
After solving the issues and assigning all these jobs to the correct users, you can delete them.