The Collaborative International Dictionary of English v.0.48:

Unbishop \Un*bish"op\, v. t. [1st pref. un- + bishop.]
   To deprive, as a city, of a bishop; to deprive, as a
   clergyman, of episcopal dignity or rights. [R.] "Then he
   unbishops himself." --Milton.
   [1913 Webster]