Presuming that you need to do this (your cat has flea issues or is a long-haired cat with a tendency to get matted), about the only way I've found effective with this is persistence.
My technique is to start by giving the cat the preferred type of grooming until the cat is happy and relaxed, then move to the area the cat dislikes. I try to use one hand to "catch" the cat's paws so the other can reach the targeted area.
If the attack is a play/scratch my chin attack (no claws, the front legs pull your hand to the cat's chin/chest), I'll scratch and gradually lift my arm so the comb can get in and work the cat's belly (this is tricky, but can be done). If it's the protect/kill reflex I withdraw and give the cat no further attention for a while. When the cat has calmed, I'll try again starting with the preferred grooming.
This method has let me get to the belly of cats who preferred not to have their stomach touched, but it's not one I'd bother with unless there was a clear need for it.