You can't manually change a worker's status to 'inactive' in Admin like you can in Provider Portal. To end a worker's access, remove all the roles allocated to them:
Go to Administration, then Security, and select Users.
Locate the user record by browsing or using the filters.
Click the User's Name.
Click Edit.
On the Roles tab, remove all the allocated roles by selecting them in the right-hand pane and clicking Remove.