With the WP User Manager Security addon version 1.3, a new feature was added to disable users even without deleting their account which prevents them from logging in.
To do such, proceed in your WordPress dashboard as an admin and head to Users > Settings > Security. From this tab, enable the option labeled as “Allow users to be disabled”.
Upon enabling this option, you can disable the user from the users list by hovering over their username and clicking on “Disable”.
Once a user is disabled, they will be prompted with the default message ” Your account has been disabled. Please contact the site administrator”. Like so:
The administrator will have the capability to enable their account again from the same section where the “Disable” button will be changed to “Enable” and can be clicked to enable again.