I used:
<setattribute xpath="/entity_classes/entity_class[@name='zombieTemplateMale']/property[@name='CanClimbLadders']/@value">false</setattribute>
results in the error message:
SetAttributeByXpath (/config/setattribute, line 2 at pos 2): Patch element does not have a 'name' attribute
So, it's telling me the patch element needs a 'name' attribute? I'm not entirely sure what that means.
Edit: I just went in and changed the value to false in the main xml and zombies still climb ladders, so I'm inclined to believe that it's hard-coded or otherwise bugged in some way, and that for the time being their ability to climb ladders is unchangeable.