An extreme difference in team should let a simple majority forfeit
I hate FF spammers as much as the next person, and there's absolutely no reason to forfeit a game that is still mostly even or even where one team does have an advantage over the other.
However, there is a point when a game is unsalvageable, and it feels terrible to be trapped in a game 8 kills to their 33, unable to do anything to change the outcome of the match.
There should be a point, an extreme point, mind you, where a team can forfeit with a simple majority. A 3 to 2 in favor of surrendering should go through if there is a huge difference between the teams. This limit would take into account team gold, objectives, towers, kills and deaths. There has to be some mathematical difference in teams that even an AI can recognize that the two people voting "No" are just stubborn and are willfully keeping their teammates in a game that they have no chance of winning.