Payment instruments that should be set as reserved are deletable.

"The common payment methods are installed by default and cannot be modified (e.g. Check, Cash, Credit Card...). If your site requires additional payment methods, you can add them here."

http://drupal.demo.civicrm.org/civicrm/admin/options/payment_instrument?group=payment_instrument&action=delete&id=47

The admin interface allows you to modify and any payment instrument. I imagine this might cause problems and that some of them should be recorded as reserved (though it would still be good to be able to disable and perhaps rename them even if they are reserved).