Hi
granite paving slabs can be very porous if the oil/fat/grease has been on it for some time then your fighting a losing battle as it will suck its way through the slab itself.
If you get it quick you may stand a chance of removing it.
A little cheap trick i use,and this works really well with concrette slabs is to mix up a paste(peanut butter thickness) of washingup liquid and washing powder,dampen the slab down with water then brush your paste on nice and thick,trick here is not to let is dry out if it does the paste will dry to a powder and be worthless so try and cover it with some plastic tape down the edges,leave it for a day or so and go back to check you may have to repeat this,its worked a few time for me,it all depends on the slab itself.
Wash off with hot water,you got no chance with cold,let it dry see how it is,if no luck after a few trys forget it,see if the slab can be replaced.