Then the only way as someone mentioned, is boycott season tickets. Except people are too scared to do it. A smaller club can organise things better. We are so big it is hard to co-ordinate this sort of thing, same with people not buying tickets on a game to game basis. Maybe the supporters clubs need to get together and all agree not to go to games on a certain day. Trouble is there is only the City game that would have a big impact.
How about going to the games and just chanting 'Fu** of Glazers' throughout the entire match, especially the ones which are live on TV. The Premier League would be forced to take action, perhaps even having games played behind closed doors, the TV companies would be reluctant to select United games for live coverage, and sponsors would start to question whether their association with United was positive. United remain a big draw for the TV companies, and it is probable that the amount of money companies would be willing to pay to secure TV rights would go down if they no longer felt broadcasting United games was viable (which would upset every other club in the league).
Perhaps some green and yellow banners and scarves with the letters FOG on them would add a bit of visual impact to proceedings.
The Glazers will be reluctant to walk away while the club remains a cash machine, so we have to create a scenario where it ceases to be one. Harming TV and sponsorship income, and causing damage to the brand, would do just that.