"Delete members now" button

Implementation:

This could be implemented via a button that makes an AJAX call to a Joomla administrative backend URL where the module is published by default, after the module is converted to a backend module. It can't easily be done with a frontend module, because I'd either have to make a frontend component to run the delete code, or require the administrator to first log into the frontend, then click the button in the backend, so that the delete code would actually run (it only runs when the page containing the module is loaded by a logged-in administrator.) But an administrator would always already be logged into the backend when they click the button in the backend, so a backend module would work.

Here is how to extend Joomla's parameter types to display arbitrary HTML:

http://docs.joomla.org/Adding_a_multiple_item_select_list_parameter_type
http://docs.joomla.org/Creating_custom_template_parameter_types

A user writes,

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Ideally it would be a button that would detect how many users the current settings would find, and display that: "14 users found and marked for deletion. Continue? [Yes ] [Cancel] " or something like that.
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I can make an AJAX call to an admin backend page, and add a URL parameter which the module can check, and only then output info specially for the purposes of displaying that message, but it may be hard to parse that info (the HTML of the whole page of the Joomla backend) into a suitable message in JavaScript; we'll see.