if it has gone into the clear coat, then nothing you can do liao. :P however, if it is still on the surface, you can try something something like what they mentioned, bug and tar remover, else you would like to try DACP and if that doesn't work, then try DCC. :P
i have a couple of spots which are not removable too. even with machine and DCC. next thing to try would be sanding and even I doubt that would help. Haha. Lesson here, is to wash off bird shit as soon as possible. :P