Customising the "Add Block" form

Hiya,

Can anyone advise me on customising the Add Block form?

I want to hide the "User specific visibility settings" and "Role specific visibility settings" from users. This is what i've got so far, but obviously it's not right and I can't figure out what the array is. Anyone got the experience on this?

function theme_add_block_form($form) {
$form['roles']['#prefix'] = '

';
return drupal_render($form);
}

Thanks,
H