The Collaborative International Dictionary of English v.0.48:

Unsubstantialize \Un`sub*stan"tial*ize\, v. t. [1st pref. un- +
   substantialize.]
   To make unsubstantial. [R.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

unsubstantialize
    v 1: render immaterial or incorporeal [syn: immaterialize,
         immaterialise, unsubstantialize, unsubstantialise]