Contracts can be terminated. If a player starts sulking, making disrespectful comments etc, lower the transfer fees and put him on market. get a replacement who's willing to play, move on. Simple.
Note that players don't just start making a move to leave a club just like that, before any move most expecially when it gets to the public domain and becomes a matter people starts giving input on, it's likely that it's been considered internally and no meaningful resolution was gotten via that means. It's same way with players also, imagine that a club wants to sell off a player even when the player wants to stay for a longer duration of time, do you think the club will consider the players interest that much? They will go on to sell the player and get whoever they want. It's even good that players are lot standing up to make clear statement on what they want and the team they want to play for so they don't end up being thrown to any direction as though they don't have any choice.
If as a player you're not comfortable playing for any team, I'm totally in support of them going off that team to a place they want to be. It's that simple.