If you are quick enough, pick up a zombie and throw it in the river or water. It will vanish.

Use Holy Water Arrows to defeat zombies. Holy Water can be found in fonts or bottles. Bottles are displayed in your inventory and can be purchased at the beginning of your mission. You can also pick some up from fonts or elsewhere in the game. Shoot the zombies twice and they will blast apart. You must be fast with Holy Water, as it turns into ordinary water after thirty seconds, after which it has no effect on zombies. Use it wisely. The only weapons that work against zombies are Holy Water Arrows. Nothing else works. If you hit them with you sword, blackjack or anything else, they will fall onto the ground, but will get back up again. If you find yourself helpless against one, quickly escape by running around until you jump up a ledge, go through water, go up ladders, or go up lifts.

Contrary to popular belief, Holy Water arrows are not the only way to kill zombies. Blast them with two Fire Arrows and they will explode in the same way as they do with Holy Water.