Telling someone 'no' doesn't have to be impolite. Saying no and explaining why, is a way to make it sound like you want to help, but there's no benefit to either side in going down this route.
Going back to the original issue...
I work in an ad agency. We commission photography a lot. Not as much as we used to perhaps, because our own clients don't always see the value in commissioned work, but that's another digression.
Our normal way of working will be to agree a number of shots upfront - these will be delivered fully retouched.
The actual amount is staggeringly small given that there may be thousands taken during a shoot.
The process may be that we view unretouched selects, make our recommendations, and then depending on the level of trust we have with OUR clients, we either instruct the photographer to process the shots, or await client approval.
The agreed amount will be retouched and supplied, anything 'extra' will be negotiated after the fact.
there may be some to-ing and fro-ing on the selected images, but this is often when you're dealing with people as models. Their hand looks odd in that shot, their smile is a little crooked etc.
What I would say is that I can't think of any occasion during 25+ years of working in this way, where we've asked for all the assets and found anything 'better' than the supplied images.
Admittedly, I've not done loads of food photography, but the shoots I've been on, we agree the angles on the day and shoot those. The rejects might be ones where the flash didn't fire in sync and would be unusable anyway, but this is all a learning curve and maybe they don't understand it.
With food photography, I can't see the benefit in asking for more. The only scenario I can think of is if you'd rotated the food and took shots from different angles, then rejected that angle for one reason or another - either on the day or in PP. And they think they'd be getting more variety this way.
So I genuinely can't see what they'd be gaining. Or what they think they'd be gaining.
I think, perhaps a little explanation of the processing process might help your clients understand why they can't just have 'more'. Explaining that you only process the ones you think are worthy and that anything else is in an unusable state might help sway it.
You could offer to come in and show them the rejects (on your laptop) with a view to coming to an agreement for post-processing and supply of any new images.
But, I definitely agree with
@Phil V that supplying them any new images should be an absolute no.
One thing I can almost guarantee is that if you supply your worst image as a way to try and convince them not to use it, they'll use it...