Every restorer comes across thick body filler somewhere on the car restoration project they are working on. Body filler can be sanded off, ground off or chemically stripped, but all three procedures are incredibly messy and two of them (sanding and chemical) are expensive.
Over our years of car restoration, we've experimented with various forms of removal and found one to be extremely cost effective and rather quick, plus there's far less mess. What you need is an ordinary propane torch with one of those "fan" attachments on the nozzle (every torch kit has one.) You'll also need a putty knife or drywall taping knife to do the scraping/lifting.
Crank up the flame and hold it against the painted/filled surface. Within seconds the paint and filler will bubble and burn. As soon as it does you can scrape it away with moderate pressure on the blade. As the blade gets hotter it will sink into the surface and help remove material. Flaming paint/filler will fall to the floor, so keep a spray bottle of water around or wet the floor. Cleanup is easy, just sweep the chips and flakes into the trash.
We've found removing filler up to 3/8 inch thick to take a couple passes, but an area of roughly two square feet can be removed to the metal in less than 10 minutes. As with all our car restoration processes, safety is a consideration. Do it outdoors if you can, or have an exhaust fan running. The smoke builds up somewhat.