Can we clear up a few things? Them threatening to report you is indeed a threat and can be punished.
But why are they not punished? Let's have you understand the behavioral system a bit more. Think of it like a health bar (or tension meter) in music games. When you hit the note at an off beat, your tension meter increases. When you completely miss a note, the meter increases by alot. When you hit the note at good timings, it reduces the meter.
Riot's behavior system works the same way, as in we all have a toxicity meter attached to our accounts. When players are mildly toxic, it goes up a tad. When people use hate speech, it pretty much fills up the meter. of course, clean games reduce the meter.
You think they are not getting punished, but they are accumulating toxicity points in their behavioral history. When they reach a certain threshold, they will get punished.
So what about their threats? It actually does nothing as 1 report does the same thing as 9 reports. All a report does is to flag the system to review the game. The toxicity score returned from the review will be the same regardless of how many times a player was reported within the same game. It is also possible for the system to return a toxicity score of 0 if there was no toxicity found within the game.
So don't give them a reason to make their threats a valid report.