One thing to keep in mind is that zombies tend to be attracted to noise and a full combat event, even with only melee weapons, makes a fair amount of noise. Keep adding more and more zombies to the mob each round until the players are simply buried under the crush of bodies. As shown in numerous zombie movies (and tested by the Mythbusters ), strength of numbers will eventually win out.

Added environmental penalties make things even more deadly. Normal debris and dispatched zombies will clutter up the area with loose bits of trash and slick with unpleasant fluids. And don't forget the stench of the zombies themselves.

As mentioned before, make note than any physical trauma inflicted by a zombie could easily lead to infection, death and rising as a new zombie.

In game terms, sooner or later you will be rolling so many dice with so many bonuses that there will simply be no way the players can survive except by running away before things get to bad for them.