What if all the tributes simply refused to fight?

I think they would have killed their family members or loved ones to get them to participate. I think Haymitch had alluded to that in one of the movies; it was a way for them to even control the victors.

Remember some of them are "professionals", people who are eager to join up and win wealth and fame.

They're not going to agree to strike, and they'll kill anyone who does.

Also, if there isn't enough killing happening, the gamemakers introduce things into the game to beef up the action. Remember the fireballs being thrown at Katniss? There were also those wild animals that chased her and Peeta towards the end.