Its been my experience that someone who would gob on a load of filler without first at least trying to hammer and dolly the surface close to original probably didn't spend a lot of time on surface prep before applying filler.
If you can remove the panel and work on a bench, or have access to the back side, a rap with a hammer from behind will usually pop the filler, or at least send the sheet metal back toward original, leaving broken edges of filler you can get a good thick scraper under.
While it amazes me how much backyard band-aid bodywork goes on, the great part is.. its usually easy to remove.
As a last resort I have used an oxy-acetylene torch to heat and soften filler making it easy to scrape. Standard open flame precautions should be followed.
