I had the same issue. I'm writing here just so (hopefully, but unlikely) someone at Apple will take this issue more seriously. I had to call Apple and was informed that likely someone marked me as spam and then Apple automatically blocked me from being able to use iMessage.
She also confirmed that if just one person does this, you will get blocked, and not only that, Apple will only unblock you one time as a courtesy, regardless of the cause of the block (e.g. even if you were marked spam my mistake).
I politely explained that it is preposterous that ONE person can screw things up for someone else like this, and there isn't even a human at Apple in between to verify that fraud/spam/abuse actually occurred. There was no warning, either. One day, iMessage just stopped working. I even asked if there would be some kind of punishment for someone who abuses the report spam feature gratuitously and capriciously, but she didn't know. The rep was understanding and seemed to agree with my concerns and said she would give feedback about this issue up her chain of command.
This system seems to work a bit like the police... whomever reports/goes to the cops first is (de facto, even though authorities like the police, and in this case, Apple are supposed to be neutral) treated as the victim by default, and it becomes the other party's burden to show they are innocent. I'll call this policy what it actually is: "guilty until proven innocent".
In the mean time, now that I know how Apple works, it is best to use Apple products defensively and selfishly. If you think there's even a tiny chance that someone might report you instead of just deleting your contact/message thread from their phone (this could be simply an accident, or perhaps someone you had a falling out with in a social, professional or romantic context), you basically have to be paranoid and report them first, which is just a terrible way for humans to interact on this planet. I've had teammates in class that I didn't particularly get along with, but otherwise there was no ill will. When the quarter ended and I no longer had any reason to interact with the person, I simply deleted their contact/message thread from my phone. No prejudice, no drama. Same thing if I went on a date and the chemistry just wasn't there. "... but nice meeting you anyway" and then delete.
Did I miss the meeting where we all just agreed to act like bitter children to each other? Sheesh. Clearly that's what's going on, and since apparently I have just used my 1-time courtesy unblock, I guess I'm being forced to devolve to everyone else's level of behavior for as long as I use Apple products (which I honestly hope is a long time).
I'm sorry I piggy-backed on your post here without actually solving it, but hopefully since there's (at least) two of us now we'll get an official response from Apple clarifying their policy and ideally making it work better for humans, as their products are otherwise so good at doing.
Cheers.
