I believe this also works:

The excerpt below is set in the page settings in "Page Header Tags." It hides the UnRegister button using a style definition for the UnRegister button's ID. This ID can be found by logging in with a user other than an administrator, loading up this page (should show UnRegister button if style definition is not working), view the HTML source of this page, and search for the element which contains the UnRegister code.

It worked indeed.

For me this was de setting that worked:

Page Header Tags: Enter any tags (i.e. META tags) that should be rendered in the "HEAD" tag of the HTML for this page.

<style> span#dnn_ctr391_ManageUsers_User_cmdDelete {visibility: hidden; display:none;} </style>

the code 391 is the module_id, wich is different in any dnn-installation. That's the reason why you can't make the setting in your CSS-class when your skins are to be distributed to other dnn-installations.