To make a user inactive, search for the user in the main User menu.
You need administrator permissions to maintain user details.
Click "User Is Inactive" and Save the user record.
View the User Reports to see a list of Active and Inactive users, from the User > Reports menu.
Users who have actively worked on jobs will be linked to various entities in the database. e.g. a user may be assigned to tasks or payment authorisations.
These users cannot be deleted from the system and can only be made inactive. Only users who do not have any links can be deleted.
There is no real reason to delete a user, simply make them inactive.