I know what it's like...I had 3 spots on my Maxima that were from bird ****. The PC could not help me, regardless of what I used with it. That's why I told you that all you can hope to do is [do your best to] prevent it from happening again.
You can try a strong paint cleaner like AIO, and work it into that area for a while. If that doesn't remove the staining, it's etched into the paint. That means the only hope you have is to polish away the clearcoat until you've removed the whole etching. Whether that's a good idea or not depends on how deep the etching is. It definitely isn't worth it if you'll have to remove half of the clear to get rid of it.