You’re asking about justice. I haven’t had much of a chance to analyze justice yet, I rarely use the concept myself. The first thing that pops to mind when I think about justice is this clip from Hateful Eight.
The relevant distinction here is that justice is decided by legitimate authority. A mob is not legitimate, but the state is. I don’t want to get too far into the weeds on the concept of legitimacy, but suffice it to say, that an individual cannot decide what is justified and what isn’t. It is the domain of the collective.
This leaves a little, no I mean gaping hole called slavery. Was slavery justified? That’s a weird little question that largely depends on whether morality is atemporal or not. George Washington kept slaves, does that mean that slavery was moral, considering he was possibly the most upstanding moral paragon of the day?
You don’t write limericks about someone’s honesty unless they were either really honest or really shady, and Washington was clearly the former.
We mostly think about this in terms of flaws. George Washington was flawed in that he kept slaves. We can analyze his flaws and come to a very nuanced understanding of where Washington’s mind was as a slave-owning politician.
Let’s return to your question, now, with our newfound understanding. It’s the collective that determines whether what you do is justified. Unless I’m a crazy moron, I’m pretty sure everybody who is going to answer your question here on Quora is going to tell you that throwing food at someone is cruelty, and cruelty is never justified, no matter what outcome you’re trying to achieve.
Even if you were to accomplish your goal of getting someone who is unhealthy to eat more, the ends do not justify the means. The zeitgeist has decided that this is anti-social behavior.
That said, justice is collective, and you can change justice by changing your collective. It might have been justifed when you were a kid. It might still be justified in other parts of the world. Do you have a time machine? No? So if you want the ability to be cruel to people to satisfy your base desires about how people should look, maybe move somewhere where this is permitted and even encouraged. Might I suggest Saudi Arabia?