I'm actually a self-proclaimed psychologist-behaviorist (oh my, I love sounding ridiculous), so I've decided to share you my researches and teachings about "how to correctly react to different situations, as a proper civilized and mature person".
Since our objective consists into expressing our inner maturity without threatening our amiability/affability, let's acquire (or remind) the knowledge and mechanism of Roman Jakobson's function of language/communication.
So, in this context, you and the other person are in front of your respective device of communication, the most common being the personal computer. The consequence implies that certain "tools" are disabled or replaced. E.G., your non verbal language can be replaced by emoticons :D, CAPS LOCK, punctuation... or images.
Let's assume that communication is a martial art, practiced by everybody, your friends included. Why do I consider it as a martial art ? Because sometimes, words can be extremely painful, in a psychological perspective of course.
Our goal is to acquire the best/highest maturity possible, so it is time to show an example about how to proceed.
Imagine you're a fan (don't amalgam with fanboy/fangirl) of Bendy and the Ink Machine (or something else), just like me. All of a sudden, a random person pops out and starts blaming/flaming you, because you've (in)directly shown your appreciation of the aforementioned horror game. What do you do ?
You can politely tell this person that it is your right to express your preferences on the Internet. At first sight, you think you've correctly reacted, like a mature person. But it isn't the case. Why ? Simply because you've (un)consciously and deliberately ACCEPTED this message to reach and trigger your mind. Why is communication a martial art ? Because when someone flames you, you naturally respond. Because when someone attacks you, you take the hit in the face and "attack" back by replying.
You can also blame this person back, but then you're placing yourself down to his level (and aim). And you attack back, with the risk of hurting a potentially triggered person, or to just feed a troll.
Now you can tell an admin that someone's not being nice, but that's not a proof of maturity. Because first, it's up to mods to intervene without the need of reporters. And second, you've accepted to take a hit.
The correct thing to do is IGNORING. The Internet is not real life, there's nobody who can follow you and provoke you to force a reply. And if this person continues the blame, he'll just talk to nothing else except a wall of emptiness. Maybe you'll think you let that person win the conversation, but it's not the case ! A conversation requires at least two persons. By not taking consideration of this person, you let him alone in his monologue. He may think he won against you, but he just won a fight against no one. And you won a higher level of maturity, by deciding that you only take credit of what you desire. Nothing unwanted can reach you.
Then a mod will warn the flamer for his childish/inappropriate conduct.
This also works in RPs, if someone puppeteers your character against the rules, just ignore and wait for the game master/mod to do his job. Now if you are the game master, ignoring will work as long as you tell the other RPers to continue the game normally, it's your duty.
Always keep in mind that you'd rather allocate time to better things than to chronophagous wastes.
Now, don't boast over your newly acquired "positive mental attitude and superiority". You'd have already lost it by doing so. It's not something special, otherworldly or something that requires genius. It's something that everyone should naturally have, but people have their own way of doing things, or it's related to (self-)education, personality, current state of mind/personal problems.
If you managed to read it all or not, in both case : thank you for your attention.
Please share your thoughts, experimentations and/or questions. I'd be glad to answer !
Feel free to submit ideas of intellectual debates such as "How to keep a fandom's reputation as decent, instead of decadent".
TL;DR : ignore mean comments, focus on constructiveness.