That's the nature of blind pick. If youu want mid, pick a champ, lock it in, say mid. If someone else wants mid, it's up to them to take it and duo mid (which is obviously stupid) or to pick something else. The idea of calling roles is just like a house rule in monopoly. You have to agree on it. We're all strangers... you don't have to follow the "meta" of blind pick, just like you don't have to follow the "meta" of the game. Neither do they. If you pick mid, lock it in, and then these guys say what they say, just play the game out, have fun with it, and report them after if you think they did that without trying to win the game. THAT is what's reportable. Not what you pick... but whether you try to win or not. Just write a description in the report of what happened.
I myself only play blind pick if I don't care what role im playing. Why would you go into a chaotic pick mode with the idea that you're gonna play mid? If you wanted mid, go draft pick and INTENT pick mid. There is no intended pick in blind pick. That is literally why there is draft mode. If you want mid, but you don't wanna wait through the champ select lobby, then I'm sorry buddy, the world isn't a perfect place for you, you're gonna have to wait an extra 3 or 4 minutes.
If you disagree with me here, that's impossible. This is not a matter of agree or disagree. Show me where Riot states that you must type in chat what role you want and adhere to that. Bottom line, show me the rule.
Edit: if this was a draft pick game, then yes you're completely right and that's fucked up. I mean, you're right anyways, they are jerk offs for doing that. But it's not punishable and you have to realize that is the nature of blind pick. It's the wild west of game modes. Doesn't mean you can troll in game, it just means you don't get to pick your role the way you can in draft pick.