That’s a tough question because there certainly should be a way to refuse, but context would have a big influence in terms of the conductor’s attitude and how dependent the harpist is on the particular gig.
It is a frustrating problem because there are plenty of musicians needing work, and plenty who are capable of not being incompetent in creating parts. We have more supply than demand for musicians, and so competence should not be the issue. I can understand a composer/arranger making a few mistakes in a score, but I don’t understand composer refusal to adapt parts because a composer never stops learning.
I would turn down unplayable parts and have, but I’m not in the employ of one orchestra. The difficult aspect is if the harpist is standing alone in their refusal, because it is better to demonstrate that it is a larger problem.
Remaining calm in potential conflict lends itself to credibility, although can be difficult when under pressure. One diplomatic approach would be to include some kind of compliment about the piece, but then to add specifics about what is not playable on the harp – that the piece has potential, but is not finished, and that it deserves to be heard when it is ready and at its best in terms of playability. Each situation is different and that statement might not always reflect the current scenario or get a positive response. Perhaps it could also help to give the composer/arranger a reference or a copy of a good text for writing for harp. Even if it doesn’t solve the current problem, if it is a person getting performed, perhaps it will help for the next project.