I'm with Phillip on this one. People should be able to enhance the truth in one game and not have their actions carry over into the next game. By establishing a standard in which "person x can now never be trusted and should be blacklisted because he broke a no-trade agreement" you add too much tangibility to a game of mafia. You're holding mafia to a double standard. Either they engage in an agreement that will put them in a lot of risk or they refuse and get lynched for being sketchy.
I understand what you're saying, but don't your actions carry over to the next game regardless? Everyone knows that Daniel, for instance, is a brilliant liar who regularly plays both sides regardless of his own affiliation. When you play a game with him, you come into it with that knowledge and you base your strategy accordingly. When you play a game with me, you know I never lie, and you likewise adjust your strategy based on that. Back in the day when Esther and Elijah both played, everyone understood that they would put sibling interests ahead of either team, so if they were on different teams, no one could trust them. That played out in a couple games and it established their reputation.
It's a game, yes, but it's based on strategy, and a large part of that is knowing your teammates and opponents. There certainly shouldn't be any hard feelings about it... it's all in fun... but even if you could, you wouldn't want to relearn everyone's strategies every single game.
(Besides, no-shares don't have anything to do with the quality of the information shared. If I enter into a no-share with someone, it doesn't obligate them to tell me the truth. It just obligates both of us not to tell other players what we told each other.)
My guideline is, if you ever have to ask yourself if you're doing something creepy, you probably are. But then go ahead and do it anyway because being creepy is fun.