Crush them.

Its time to crush this zombies balls. You clamp down as hard as you can, making the zombie yelp in pain.

His balls have nowhere to escape as both of your hands squeeze each orb with excruciating ****, making the zombies legs quiver and making him tear up.

You squeeze harder, harder, and harder. His balls straining more, and more, and more... until...

POP! POP!

You hear two audible pops as his testicles burst under the pressure. You let go of his ruined balls and he clutches whatever remains in his sack before passing out.