I've noticed from the thread that people don't have a problem with off-classing, its more of a problem with constant off classing or off-classing in excess. I think i good way to mitigate part of the problem would be by having the 6s config limit off classes to 1 each. This would allow for forward-spawning and off-classing as required and hopefully cut down on off-class only defense.
That's sort of the case already. All classes have a limit on them imposed by the config. I don't think there's a way to limit classes so that only 1 pyro or heavy or sniper can exist at the same time you can only limit them to numbers of each (as far as I'm aware anyway).