I don't like to put sneaker balls or spray anything
inside.
Spraying puts in more moisture right? Then with more moisture errmm
things will grow? 
I think if shoes are smelly, they'll smell worse if instead of
eliminating the smell, you throw in something hoping it will
overpower the stink with a "fragrance"... Can you imagine
stink+fragrance?
Just sun and air your shoes frequently.
You don't need to buy special charcoal. Just use newspapers. When
my microwave has funny smells I just dump crumpled newspapers in.
