The OP sounds like someone who played a lot of Dota, and is completely right.
We have experience with these kind of things. Banlists are definitely needed, or at the least some kind of ignore list. Demigod has neither.
By the way, I banned you for being an immature moron. Don't worry, you won't be the first person to be banned just because someone is a huge dick. See what I mean? Banlist is completely stupid. Someone can be perfectly competent, and maybe his mom just died, or his girlfriend/boyfriend came, or his electric company craps out, and yet still be banned. And your computer crashed while in the lobby? Well according to you, you are a "moron" who doesn't have enough common sense to take your PC to the shop for cleanup or do it yourself. Is that fair?
Shared banlists don't work. Everyone knows this. Local banlists work wonderfully. If your mom just died, or girlfriend/boyfriend came or <insert other legitimate excuse here> (though "boyfriend came over" might not be so legitimate if you knew they were coming but started a game anyways) then I probably wouldn't ban you assuming you SAID SO before bailing from the game.
Everyone has their own personal thresholds for what they'd ban for. Performing fine all game but suddenly lagging out? I probably wouldn't ban. If you did it a few times, or lagged all the time? I probably would. Died a few times and left? A ban. Afker? A ban. Saw you again? Would mention the first ban and see what you said about it. Depending on your response I may play with you again. Left a second time? Banned forever. Griefer? Banned forever. Garbage sim speed making game unplayable? Ban.
You see, a ban from me does not suddenly mean that you necessarily did something "wrong" (as in the case of bad sim speed) but that I just do not want to play with you again.
Now consider player 2. They ban for nearly anything. You sucked? Ban. First blood? Ban. Said something they didn't like? Ban.
What's wrong with this? Nothing. They can ban people for whatever they want. If they ban you, is that a bad thing? No, because you probably don't want to play such a jackass ANYWAYS.
I don't see how this is a bad thing, it just allows each person to track people that ruin their enjoyment. As long as there is no attempt at a crappy "public" or "shared" banlist, which as I've already said NEVER works, implementing a personal banlist is only for the good of the people playing the game.