In the US version of the TV show Kitchen Nightmares, I see customers returning food for any reason. But when I am in the US, I pointed out problems with the food in restaurants. They apologized but didn't give me replacements and just asked me to pay. In the first occasion, the worker sneezed on my plate. The restaurant owner says that is normal for a person to sneeze and they can't do anything about it. In the second case, I had pizza with crust that was still dough, clearly not finished cooking. They still asked me to pay for it. Should I be able to expect replacement food in such cases? Can they call the police if I refuse to pay?
Yes, typically you can return food if you have not eaten any/much of it. I would never accept food that I saw someone sneeze on, and what you were told is nonsense. Frankly if I were told that I'd get up and leave, not paying the bill.
Legally though I don't know if there is any requirement to "make things right" or not. Certainly if you haven't paid yet (and have not eaten / drunken anything you'd have to pay for) just leaving works. It possible they could call the police, although I don't know what the ramifications of that would be. The police might make you pay, they might not, they might tell both of you to work it out yourselves. In the case of the pizza if you paid upfront, you probably don't have a lot of options.
In both cases, leaving a bad Yelp review or equivalent can always be done without worry.
Fortunately most restaurants are not as crumby as the ones you went to and will offer to remake the food at the very least, or not charge you for the portion which was done poorly. Its not the whole meal keep in mind, just the thing they messed up, unless they really valuable. I've had times where my wife's dinner was done improperly, and the restaurant comped both of our meals and offered free deserts too, but we are regulars there.
One final thing; "any" reason will not fly. Usually if something is improperly cooked, over-salted, has a hair in it, etc., restraunts will try to make right, but if you simply didn't like the dish and otherwise there was nothing wrong, most would probably not do anything.
In addition to Andy's suggestions, if the restaurant is giving you food that they sneezed on, that will be of concern to the state or local department of public health. I would encourage you to contact them and submit a description of what happened. (If you're ever in a similar situation, simply threatening that you will tell the department of public health can sometimes make a difference.)
Just to simplify, I'd say you don't need to accept food that's sneezed on and you can totally argue with the restaurant owner without fear (as @Andy said, at least if they intend to run the place for more than a week!)
Although with all good intentions, the reason I don't go over legal advice is that sometimes the questioner or whoever sees these questions later, don't live here and not familiar with how things work. Anything that smells court or police involvement, may scare someone to an extent of not saying anything and prefer a mistreat over a dispute with the restaurant!