You can refuse Bella's help though, which locks out the Bella route.
Now Jade's I don't remember much, I remember it was rather straightforward.
if you are in the Bella path you cannot reject her, you can only reject her if you have not done anything with her in chapter five, effectively putting you out of her path
the structure of Bella's Path is quite simple
go see her to thank her for putting in a good word with Jill?
If not, you're out of her story forever.
If yes, you're in
in chapter five do you push to do something with her?
if not, you're temporarily out
if yes stay in
in the sixth chapter if you are in her path she comes to help you, otherwise you can accept her help.
if you accept you are again in, otherwise forever out
then in the seventh there's Cathy