This situation has happened to me once or twice, and perhaps it's happened to you too.
You sit down with people outside your normal playgroup, and someone says, "I have [banned card X] in my deck. I don't think it needed banning, I don't abuse it." Perhaps not these exact words, but something along those lines.
Before I offer my opinion on what to do, I want to talk for a moment about "I don't think it needed banning/I disagree with the ban/etc." I've disagreed with the bans, until I did something 'normal' with them. Griselbrand is a great example of this: You don't need to force him out turn 4 to win. Play him turn 8 and you should be able to take care of that game. Same with Primeval Titan--just fetching a Ravnica bounceland and a Temple of the False God is bad enough, but when you get crafty and fetch Cabal Coffers/Urborg, Tomb of Yawgmoth or Maze of Ith/Deserted Temple, then the bans start to make a lot of sense.
Without trying to be too universal, here's the banlist criteria from the official site. The short form: A powerful card, that even used fairly, tends to lead to winning, but when used to its fullest potential, is nigh-unstoppable. I found that Griselbrand was terrifyingly amazing in any deck playing black, and that was before I did things to maximize what he offered.
The Rules Committee doesn't want to ban cards. I realize that to some people, police/moderators/teachers are symbols of authority who only want to take joy from making other people do as they wish...but that's unrealistic. The RC is a group of guys who like to have fun. They tested and developed this format, and indeed, it's an amazingly successful effort.
The banlist for EDH is the smallest of any format, except Vintage, which restricts most cards and only bans ante or 'dexterity' cards. Legacy doesn't allow 60 cards, EDH bans 56. You're free to play almost anything, except for cards which take the game from 'battlecruiser Magic' to 'is it over yet?'
It's also worth mentioning that the RC is a big fan of 'house rules' (click 'philosophy') or 'local banlists.' To them, this is a format where you're free to do the things you consider fun, and they don't want to step on that. I totally agree--if you and your pals want to play a different way, go for it. I've been part of groups that tried starting with two basic land in play, and games where you were only allowed to tutor for basic land. Interesting experiments, ultimately not worth doing. (Might end up trying the second again sometime.)
So back to person who wants to play card X. Before the game, this is just fine to bring up and ask the table for opinions. If the majority, or even a vocal minority of a table express that you should take out card X, yank that puppy out like a rotten tooth. This is a game, and should be fun. If people don't want you to play a certain card--especially if they already took that card out of their own decks--then you should absolutely abide by the group's decision.
If someone ignores what the table decrees, then you are free to decline further games with that person. I know this seems harsh, but again, you want to have fun. The RC wants you to have fun. It is perfectly acceptable for you to decline games with someone whose presence/playstyle/deck is not enjoyable to you. It is not harsh or mean, though it can feel very elementary-school. "I don't want to play with you" is quite the claim, even when worded as politically as "This is no longer fun for me."
Sheldon has become fond of pointing out that this is a social format, not a casual one. This can be a little hard to grok, but the goal of EDH is a fun social interaction, with a game involved. The game provides the context for the social interaction.
I know, and have known for a while, that EDH was the stated reason to gather friends, but the point of such an evening was not who won or lost a set of games--it was that they were played with good people, and a good time was had by all.
Cube drafting is, in my experience, much the same way. The delight is in the people you're playing with, and the sounds they make as they are forced to choose between two amazing cards. The games are fun too, but again, that's the excuse, the context for the interaction.
How out-of-place would it be if after a 4-person EDH game, someone was incredibly bitter about losing, and demanded a rematch? A rematch request is nothing new, but I can count on one hand the times where someone was so upset that they had to play again.
We've all had games where we just lose. I had two games in a row a couple weeks ago where someone landed a card that I never drew an answer to (Aura Shards game 1, Umezawa's Jitte game 2) and those were decidedly not fun for me or anyone else at the table. I had answers in the deck, and even tutors for those answers, but none of them came up and I died. This happens and this is not fun.
That situation is different from someone wanting to play by a different set of rules.
I do not encourage anyone to say "Griselbrand is unbanned in our store? Excellent!" and then build a broken deck around said demon just for the purposes of illustrating why he should be banned. That makes YOU the bad guy, being all passive-aggressive and annoying. If you don't agree to a certain rule, don't play.
Likewise, if your group or store has a local rule, be accomodating to those who don't follow it. Have one or two for that set of rules, but also have decks that adhere to the RC's rules. It's fne if some folks have a Thursday night 150-card EDH deck night, but have a couple of 100-card decks, for people who haven't gone that route. Same thing if Pauper EDH is your bag, or the French 1v1 decks. (Topics I promise to cover soon, I swear!)
As for me and the most recent time this happened, I told the guy that I had taken that recently banned card out of my deck, and I felt it was unfair for him to have one. The rest of the table was noncommittal, the card stayed in, we played, and we all lost to a Kaalia deck that went unchecked. Again, these things happen. It was a good time nonetheless.