As much as I hate mass free killing jumping to the conclusion of nerfing classes to avoid it is a little extreme in my opinion. You'd also have to end up nerfing stickies, pyro's can also have a fairly easy time at mass free killing, a sniper at a distance especially on the minecraft map can pick off quite a few people, a demo's grenade launcher in a cluster of people can be devastating, so on and so forth.
The only class I can't see being able to slaughter quite a few people before being killed or voteslayed is a Spy, medic, scout (who can still easily kill classes with nothing more than melee, it just takes him a tad longer) and engineer.
Nerfing their core class abilities for the sole purpose of preventing mass free killing would cause balancing issues. Either the melee would have to be nerfed further or they would have to be compensated in another way if balance is to remain.
So -1 from me