Do you keep giving her the $100?
Absolutely not! This isn't even a real question.
If you directly enable a person or group of people or country to harm themselves and others, you have a duty to stop doing it.
If US remittances are directly fueling the cartel which beats, rapes, murders and even engages in torture, then not only may the US cease that but it is plainly ethically bound to do so.
Yes, this will make it hard for Grandma to buy a car.
You can't fund the people who torture and rape random Mexican farmers so Grandma can buy a car.
> If you really want to eliminate the cartels make recreational drugs legal.
While murdering drug users with their own money may seem like a clever solution, you are in fact still doing murder just in long form.