Sadly, sometimes you get to be the dentist and pull those misbehaving teeth.
If their reports are poorly (incorrectly) formatted, you need to be able to show them the specification, and how they're not complying with it.
When I've had customers who couldn't (or, more accurately, wouldn't) make a change to bad data, the culprit was usually the cost of changing the existing code.
Slow, steady, even pressure in one direction (fix the code!) over a long period of time (sometimes years) can have moderate success. Good luck!