You can edit operator.css file

Look for this:

.notregistered {
background:#ddd;
opacity:.50;
filter:alpha(opacity=50);
-moz-opacity:0.5px;
}

and change it to:

.notregistered {
display: none;
background:#ddd;
opacity:.50;
filter:alpha(opacity=50);
-moz-opacity:0.5px;
}

It should work. Be sure to flush your browser cache so the .css file is actually reloaded after you make the change.

Best regards,

--
Nicolás