I think that you (or someone else) have the control on your WordPress MU and the plugin has been activated on the network, so you can’t delete it on you single website.
Either you access to WP Mu and delete it, either (if you really can’t disable it from WP MU) you delete the plugin folder from your FTP.
It’s very strange, I’ve never seen a bug like that.
I think you should delete the magIc post thumbnail folder into your WordPress /wp-content/plugins/ folder.