Methyl-ethyl-ketone, or MEK, will take it off in all likelihood and it does not evaporate as quickly as acetone. Read the warning label first. You will probably want to apply it with a rag or cheap paint brush-wear rubber gloves-and then scrape with a wide blade drywall knife. Mind the ventilation as recommended.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.