I don't get it why are you so offended on banning a faction...
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The idea of casual mode is to play some games and have fun.
I've gathered all points into one post. There are five reasons.
1. A ban/reject mechanism is the wrong solution
Like I have explained, banning/rejecting a faction is not a good solution and it's only being suggested because some decks are far too common (and either annoying, OP or both). Banning a faction is like putting on a band-aid, instead of properly treating the wound. We have to look further and deeper. Because, if we don't, and we continue to band-aid everything, the game will not improve. If we accept such a solution, it sets a wrong precedence for Gwent.
2. A ban/reject mechanism can be exploited for other purposes
Every player can just ban the faction or reject the match-up their deck is weak against, creating an unnatural win-rate. Furthermore, they can actually start optimizing their decks, knowing they will never have to face X faction for an even higher win-rate. Ironically, the opposite can happen too, when players do not optimize their decks, they might no longer have a favorable match-up (because others ban such decks), which might actually lower their win-rate. This doesn't create a balance, instead it creates more polarization. To put it bluntly: it makes the strong stronger and the weak weaker.
3. A ban/reject mechanism makes the meta worse
Because of the above (point 2), it also shifts the meta into a weird direction, creating decks that should normally not be able to thrive. At first, you might think that this promotes variety, but at what cost? You are getting super-optimized decks that can run rampart, which exacerbates the issue a ban/reject mechanism was meant to solve.
4. A ban/reject mechanism will punish players unjustly
Furthermore, as mentioned by Draco, it punish players unjustly (i.e. creates too much collateral damage) because there is a major flaw in implementing a ban/reject mechanism. What about those players that are actually trying out a different deck (home-brewed or otherwise)? Can you spot those? No, not always. So, if Nilfgaard does get banned by everyone, for example, then that means that no one can make another non-meta NG deck. Thus a ban/reject mechanism actually adds more fuel to the fire, instead of less.
5. A ban/reject mechanism might be hated more (then what it tries to solve)
The last reason is somewhat of a paradox. But, if players truly want to have fun (i.e. chill out) in casual, then they shouldn't play (annoying) meta-decks. Yet, this is still happening. So, players have another idea about what casual should be. Because of this, we should be careful with changing casual. I don't know how many people are annoyed by faction X, but it may very well be that more players will get annoyed when their faction gets blacklisted.
6. [Bonus] No other CCG has done this either because of good reasons.
Can you now see why it's a bad idea? If not, try to refute my points.