How and why is she hassling you for treats?...
It should be a very clear consequence, she’s done something = reward. Yes that can sometimes mean a bit of, hmm dunno what I did there to get that, but hassling? Sounds like more than that.
Also bear in mind reward doesn’t automatically mean treat, if you’ve got a very food oriented dog it might be mostly food, but especially inside with easy training, that could just be part of a meal... but it could also be praise, a toy, a game...
“Surely it’s fine to say ‘no’ in a fierce voice?”
But why? She’s a dog, she has no clue what no means, half the time people who speak English don’t know exactly what no means... what does saying it in a fierce voice achieve and teach her? That you randomly shout at her?...
Ignoring bad behaviour isn’t always just... ignoring... it’s more about not punishing so what they learn is what to do instead, not just that you get angry.
So as an example...a dog that jumps up, you don’t let them just do that, you turn so they’re back on the floor, but without giving them the attention they’re after, some dogs will stay down, they then get the attention, until they jump back up and then you repeat, if it’s a more kangaroo like dog, teach them something that’s incompatible with jumping up, a sit, so when they bounce you deflect them and get them to sit, then they get the attention.
It does depend on what they’re doing though, how you’d interrupt it and what you’d then do.