If its hard and brittle, you might be able to chip it off. Ive removed tar type sound deadening crap before by freezing it with dry ice then chipping it out too. That might work if its kinda soft.
paint remover that you can get at home depot or such. Paint it on with a brush and then cover it with visquene for about 1/2 an hour then just wash it of with a hose or scrape it off with a putty knife. Where gloves and eye protection the **** will burn you. ( the visquene keeps the remover from evaporating to fast and gives it time to really soak in)