The Collaborative International Dictionary of English v.0.48:

desex \desex\ v. t.
   to make infertile, especially by removing or incapacitating
   the sexual organs; -- used of both males and females.

   Syn: sterilize, unsex, desexualize, fix.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

desex
    v 1: make infertile; "in some countries, people with genetically
         transmissible disabilites are sterilized" [syn:
         sterilize, sterilise, desex, unsex, desexualize,
         desexualise, fix]