The Collaborative International Dictionary of English v.0.48:

Unspell \Un*spell"\, v. t. [1st pref. un- + spell.]
   To break the power of (a spell); to release (a person) from
   the influence of a spell; to disenchant. [R.]
   [1913 Webster]

         Such practices as these, . . .
         The more judicious Israelites unspelled. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unspell
    v 1: release from a spell [ant: spell]