Please don't buy an electric shock collar or spray them with water. All this does is reinforce the experience that bad things happen when this other cat is around. Hitting them/tapping them only serves to harm your own bond with them as they wont understand why you're hurting them.
I have this problem currently with two of my cats that previously got along fine. My once gentle and sweet kitty now attacks the younger one viciously any time she see's her, unfortunately. What you need to do is keep them separate at all times and slowly re-introduce them to each other.
Start by spending time with them one after the other so that you smell of the other cat each time you go to pet them, etc., then swap blankets around room to room so they become used to the smell of the other cat without seeing them. Once you've done this for a few days you can start feeding them on either side of a closed door, so they are close by each other and can smell one another but are given a pleasurable experience ie: eating. Keep doing this until you can have the door open and you sat between them with your legs as a barrier for safety reasons. Once they get used to seeing each other whilst eating it will build up more pleasurable experiences associated with being around one another. Keep doing this until you can trust them to eat together without you sitting between them, etc. You should get the gist of it. Basically reintroduce them as if it were a whole new cat but go slow. It will take months and hard work. Do not leave them alone together until you can 10000% trust them or else you will undo all your hard work.
...Should I go back to school?