You give your sister $100. You find out later that she uses $50 of that money to buy heroin and inject it in herself.
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.
You pay taxes in the US? You are directly enabling a country to harm others.
You ever buy a phone? You are directly enabling a chain of groups to harm people (in the name of creating that phone).
You ever give a dollar to a homeless man? You might be directly contributing to them harming themselves (or possibly just letting them buy food/drink).
Heck even your example about your sister, I think some people would argue that it's still the moral choice.
Clearly there's a spectrum here. Black and white thinking and statements aren't helping and shouldn't be the basis for policy decisions.