The Collaborative International Dictionary of English v.0.48:

Denaturalize \De*nat"u*ral*ize\ (?; 135), v. t. [imp. & p. p.
   Denaturalized; p. pr. & vb. n. Denaturalizing.] [Cf. F.
   d['e]naturaliser.]
   1. To render unnatural; to alienate from nature.
      [1913 Webster]

   2. To renounce the natural rights and duties of; to deprive
      of citizenship; to denationalize. [R.]
      [1913 Webster]

            They also claimed the privilege, when aggrieved, of
            denaturalizing themselves, or, in other words, of
            publicly renouncing their allegiance to their
            sovereign, and of enlisting under the banners of his
            enemy.                                --Prescott.
      [1913 Webster]

   3. same as denature.
      [PJC]

WordNet (r) 3.0 (2006):

denaturalize
    v 1: make less natural or unnatural [syn: denaturalize,
         denaturalise] [ant: naturalise, naturalize]
    2: strip of the rights and duties of citizenship; "The former
       Nazi was denaturalized" [syn: denaturalize, denaturalise]
       [ant: naturalise, naturalize]