I hate the hill assist. It caused me to stall my car twice while trying to pull into my garage. I tried to go and the car wouldn't because the hill assist had activated then when the car was finally ready to move I wasn't and it stalled.
I have figured out how to keep it from engaging though. If you are on hill keep it out of gear and the brake pressed until you are ready to go. For those that are well versed in take offs on hills it should be a piece of cake for you.
Keeping a car from rolling on most hills is easy if you know how far you need to release the clutch for the car to go forward on its own without using the accelerator. Once you understand that when on hill all you have to do is release the clutch to that point and the car won't roll, you won't have to rev it, and won't have to use the brake to keep the car from rolling back when you are ready to take off.
When I'm on hills the only time I've ever used the brake is when I pull it out of gear waiting for a light and when I know the light is about to change I would put it in gear and do what I described above.
Hopefully everything I said makes sense.