You can now delete your PHG circle by sending the PHG extensions with zero power, zero gain: 'PHG0000'. Some users have been testing PHG with their mobile station's callsign, and need to hide the old circle after finishing with the testing.
If you simply stop transmitting the PHG data, it will still be present in the aprs.fi database. There are a bunch of digipeaters using multiple different comment texts in a round-robin fashion, and some of them only transmit the PHG extension once per three position packets. So, if I'd delete the PHG data from the database whenever the PHG extension is missing, those digis would only be shown with a PHG circle 1/3rd of the time.