I think that when it comes to clanners or singles, it's very simple:
Your side gets a pick and their side gets a pick. Is there not already diminishing returns built in for winning a certain scheme frequently? If so, it doesn't make a lot of sense that someone (or a clan) would pick their best scheme consistently, unless their aim is to slowly amass wins/points through loads of games. (Maybe there should be some consideration as to how many points schemes are awarded when they are frequently picked? Are the diminishing returns currently in place enough to compel people to play all schemes?)
For clanners, I don't think any discussion of schemes should be done beforehand whatsoever. I feel like if you're willing to step up and start competing in a team environment, you and everyone else in your clan needs to be as much of an all-arounder as possible. I don't understand how people can play for years and NOT be an all-arounder. To be good at anything in this game, it takes time and effort, and pretty much every single one of us has spent a ridiculous amount of time playing this game. If you're lacking in a particular scheme, it is because you haven't put in the work. There's a couple of schemes that I definitely need to work on - and do.
I don't think that 'banning' schemes is an answer for anything. People should be forced to play those schemes they don't like, the schemes they're not so great at and fume about losing so that it motivates them to train and overcome. Too many specialists these days, and not enough all-arounders.