Source : Webster's Revised Unabridged Dictionary (1913)

Unteach \Un*teach"\, v. t. [1st pref. un- + teach.]
   1. To cause to forget, or to lose from memory, or to
      disbelieve what has been taught.

            Experience will unteach us.           --Sir T.
                                                  Browne.

            One breast laid open were a school Which would
            unteach mankind the lust to shine or rule. --Byron.

   2. To cause to be forgotten; as, to unteach what has been
      learned. --Dryden.

Source : WordNet®

unteach
     v 1: cause to disbelieve; teach someone the contrary of what he
          or she had learned earlier
     2: cause to unlearn; "teach somebody to unlearn old habits or
        methods"
     [also: {untaught}]