You could hide the Button with Javascript.
in the detailviewdefs.php include your script
'includes' =>
array (
0 =>
array (
'file' => 'custom/modules/Leads/button_hide.js',
),
),
With something like:
SUGAR.util.doWhen("typeof $ != 'undefined'", function(){
if($(#status).val() == "Converted")$('#delete_button').parent().hide();
});
(SUGAR.util.doWhen ist the SugarCrm onload if ajax ist enabled)
if the Field status is not in your view you will need to add a hidden field in detailviewdefs.php and set the value with smarty code like:
<input type="hidden" id="status" value="{$fields.status.value}">