At what point can I just say "NO"?

So I’ve only been at this company for a year, so I’m hesitant to ever tell someone no when they have a new task. BUT, there are certain people that are just impossible to work with.

They never give any solid requirements for the task up front, so I wind up doing the same thing in 5 different ways every single time. They assign me to debug their own broken (and poorly written) code quite frequently which would just be quicker and easier for them to do since they wrote it to begin with. They can never just take my word for anything. Every time I run into a bug in their code, I come back and say “Hey this doesn’t do what you said,” and the answer is always “Are you sure?” I spent a week trying to tell someone that the JSON from the customer was missing tons of data because when I showed him he would ask “Why is stuff missing?” as if I didn’t know how to parse a text file… The obvious answer was “How the f$%& should I know why it’s missing? Maybe ask the guy that gave you this file???” And to top it all off, these guys constantly push breaking changes without warning and they’re almost always unnecessary. It ruins the whole workflow when I have to go back and get their stuff (for which there is no documentation) working.

So, at what point can I just be like “Nah. I don’t know why your stuff doesn’t work. That sounds like a you problem.”