Here's a quick and dirty script to disable UnRegister. I created a custom User page. Added the User Account module. Then I added HTML text module below it. I believe it must be below it in order the javascript to work properly. After adding these two modules, I modified the text of the HTML module. I had to click the source button on the HTML editor and paste the code below into it then click update. If I use the HTML WYSIWYG editor of the HTML module, then it messes with the script.
I would appreciate suggestions if there is a better way to do this. Note: you will need to substitute "dnn_ctr518_ManageUsers_User_cmdDelete" with the id of the <span> element that contains the UnRegsiter code. This id can be found by logging in with a user other than an administrator, loading up this page (should show UnRegister button if javascript is not working), view the HTML source of your user page, and search for the <span> element which contains the UnRegister code.
<script type="text/javascript" language="javascript">
var theSpan = document.getElementById("dnn_ctr518_ManageUsers_User_cmdDelete");
if(theSpan != null)
{
dnn_ctr518_ManageUsers_User_cmdDelete.innerHTML = "";
dnn_ctr518_ManageUsers_User_cmdDelete.innerText = "";
document.write("UnRegister is disabled. Please contact the site administrator if need help with your account.");
}
</script>