I'm going to be blunt here: it's a habit that needs to be broken! Simple as that. There is no low-calorie food you can shove into you all day long without having negative side effects. So the only really sensible option is to take the sentence "I am a type of person that has to be chewing something" and prove yourself wrong!
I don't know what you do all day, but here are some suggestions to help you break that habit:
If you like to munch in front of the TV, when your hands are idle, someone here (sorry, I forgot who it was) buffed her nails instead of putting food in her mouth. Or try knitting, that keeps your hands busy, too!
If you want some snack while you're at the computer, try having a glass of water or herbal tea close by
all the time. That was it for me - my automatism was to reach out for something all the time when I was at the computer - now it's a glass of water I reach out to.
If it's while you are out and about, try taking nothing with you - and no money, either, so you cannot buy something to munch on. A week should basically do to break you from the habit.
Good luck!