Are you saying no one actually believes you turn into a cat but it has some other meaning apart from the literal meaning of those words?
<sarcasm> Cool cool cool... Back to the topic of that juju which makes you stab proof. I'm sure you don't mind if I stab you while you hold on to it. Would that be okay with the hierarchy of being and the hierarchy of meaning? Not proving anything mind you, i would never ever do anything as violent as trying to force my materialist worldview on you. </sarcasm>
That being said probably juju which is supposed to bring you diffuse and probabilistic benefit is dime a dozen, while ones bringing you easy to verify, concrete, and immediate benefits are harder to find. Such is life I guess. :)
People in traditional societies do not think physicality or empiricism is the primary way of determining what is true.
These people are showing you that fear, desperation, and hope lead them to believing a narrative that requires them to take another human being's life in the hope of gaining some physical advantage for themselves. They are literally sacrificing another human being for the hope of improving their own future.
Is what they are doing based on some empirical evidence? No. Are the effects of what they believe still real? Yes. Their hierarchy of ontology has a coherence of its own and allows their society to function, and is a very ancient structure. The concept of the "evil eye" (which is tightly related to these African practices) is a very old mechanism for managing wealth (and good or bad fortune) in communities for thousands of years.
To come along and say "well, this is nonsense. Let's convince them they're wrong by using physical evidence to prove it!" seems pretty hubristic.
Otherwise, no. In this particular case, you'd simply be trying to get people to stop killing innocents in the name of spiritual/religious beliefs; something that is clearly evil and, in my opinion, so is anyone who tries to pretend it is not whether they believe that honestly or is just arguing in bad faith.
There's also a false dichotomy here: you don't ether have to have respect for base superstition or subscribe to materialism. There's plenty of room for philosophies that reject both horns of this dilemma.