The Collaborative International Dictionary of English v.0.48:

Unchurch \Un*church"\, v. t. [1st pref. un- + church.]
   1. To expel, or cause to separate, from a church; to
      excommunicate. --Sir M. Hale.
      [1913 Webster]

   2. To deprive of the character, privileges, and authority of
      a church. --South.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unchurch
    v 1: exclude from a church or a religious community; "The gay
         priest was excommunicated when he married his partner"
         [syn: excommunicate, unchurch, curse] [ant:
         communicate]