In the User Maintenance window, the delete button is only active right before you save and commit the changes in Stat.
For example, if you are entering a new row into the Stat User Maintenance window and you decide you don't want to save it or made some mistakes, the delete button will be active. But once you save the row "Apply" button, the delete button will become grayed out.
You can uncheck the box under the "Active" column for the users that no longer need to login to Stat then set a filter if you don't want to see the list of those users displayed.
This is the case for almost most places in Stat, for example, Service Domain, Environments, Object Types, Users, etc.
You can mouse click on the "Filter" tab and set a filter similar to following and that will only display a list of active users:
status_cd = 'A'