Ah, the worst bug in Fallout 3. They never disappear, ash/goo piles are permanent. I hear they can also inflate your save file as the piles add up, and end up causing more frequent crashes and graphic glitches.
If you play on PC (unlike me, lucky bastard), you can download a mod that makes them vanish, or allows you to erase them. Not sure what that mods name is though.