You can tell someone “you suck, go back to the corner of the Internet you came from, I don't have time for this”, or you can tell them “your skills don't seem to be quite at the level where we can accept your contributions; in particular, you don't understand X, Y, and Z very well and it shows in your code. I'm short on time, but if someone else has time to explain why that would be great.”
The difference is almost immeasurable between those two interactions, even though they are saying the same thing.
I've made this point before regarding derision, which I think is the true issue. I'll quote myself, since I've already written the thing[1]:
“Derision is not curtness. It is not impatience. It is explicit, intentional putting-down of someone because they aren't as good at something as you are or think they should be. And it is a poison. Unfortunately, if you take it frequently enough, it becomes habit. It becomes second nature. And you stop noticing it's poisonous. And so, unfortunately, it is an oft-enjoyed poison in intellectual circles. It doesn't help anything. It doesn't help you understand the world or other people. It doesn't really save you that much time. It simply makes you feel better about yourself.”