I agree! It is not rude at all to send something back if there is something WRONG with it. If you just don't like the food cuz it's something new you haven't tried, or something... no don't send it back. But if you ordered a medium steak and it came rare, or well done... then yes, you as a paying customer have a right, and should exercise that right, to return the product that you are paying for.
To take your money for a steak they promised will be medium, but then delivered as well done, is false advertisment on their part. However, it is up to you to fix this, and can be done so easily.
Anyone who spits in someone's food is low life trashy scum. Period. They are not worthy of employment at even a McDonalds let alone Disney World. I would fire them on the spot, and prosecute them. You can actually be charged with assault for doing such a thing as spitting in someone's food. As the attack can be pushed as an "attempt" to spread disease or illness to another person. The first case of a successful prosecution for this was after an inmate spat at a police officer. So yes, if caught, serious charges could also be filed against the employee. As an employer it would give me great satisfaction to prosecute the kind of scum who would do something like this.