The hill assist is annoying IMO. When I first got my STI I didn't realize it had hill assist and stalled out a few times before I figured out what was going on. I can understand the thought behind it especially for drivers that are new to MT. There is a way to turn it off, like Sabrewings said it should be in the Driver's Manual but if I remember right this is how you do it:
1) Press and hold Traction Control button (Left of steering column next to side mirror controls)
2) Keep holding until the indicator light on the dash appears, turns green, turns yellow, turns off (Should be about 10-15 seconds)
3) Let go of button
4) Within 3 seconds press the button again
5) You should now see the hill assist indicator appear on the dash saying it's deactivated.
Do this or just keep it in neutral when you're on a hill since it only engages when you're in gear, I believe.
Edit: I just re-read your post, if you have the indicator light with the car on the hill with a line through it, that leads me to believe that the hill assist has already been deactivated, someone please correct me if I'm wrong though...so of course in theory the hill assist shouldn't be holding you there...