I don't think its beneficial to continue arguing about the problem when I'm fairly sure we can all agree on the problem. Like has already been said, we need to discuss the solutions to the problem rather than the problem itself.
The following quotations are from posts suggesting potential solutions. Try responding to these:
It doesn't need to be a permanent ban ... nobody is saying that. Just that if they received a temp lobby join ban maybe they would learn not to do it again. what about banning them from Mumble lobbies? (is that even possible?)
I think people are kind of looking at this from a weird perspective. Why not do some checks to ensure that people are in mumble for the mumble required lobbies. These checks could be similar to the checks to ensure that they are in the tf2 server. They would need to be more lenient of course because of the situation. The action taken as a result of not being in mumble could vary widely as well. It could kick them from the lobby and request a sub, affect their reliability rating, or even have a separate reliability for mumble attendance. Feel free to discuss.
There's clearly an icon for mumble required lobbies just as there's clearly an icon for which class you're assigned to. We don't see many people playing the wrong class in a lobby because they get punished and banned for it. Just because a large amount of players aren't joining mumble doesn't mean we should ignore it unpunished. This completely ruins the game experience for both teams. The winning team ends up rolling and being bored and the losing team gets frustrated. We aren't requesting that they get banned from the site entirely for it, maybe blocked from joining mumble required lobbies for a time frame? You can make a lobby that doesn't require mumble if you are so against it.
@fraac: that's what they're planning to do, but there are some aspects of it that aren't that obvious. Also, it's not that useful to report someone who isn't in mumble if half the team isn't there. Ideally, the situation wouldn't even arise (which would be better without complete banning).
I would suggest having a temp ban from Mumble lobbies. They still show up, but you can't click them. Then, you somehow get back that right by waiting, or, a personal favorite, by subbing lobbies.
To begin, you should write a guide to explain why mumble is important in lobbies. This is perhaps because players don't know why is that very important.