VERB (2)

1. cause to disbelieve; teach someone the contrary of what he or she had learned earlier;

2. cause to unlearn;
- Example: "teach somebody to unlearn old habits or methods"

The Collaborative International Dictionary of English v.0.48:

Unteach \Un*teach"\, v. t. [1st pref. un- + teach.]
   [1913 Webster]
   1. To cause to forget, or to lose from memory, or to
      disbelieve what has been taught.
      [1913 Webster]

            Experience will unteach us.           --Sir T.
                                                  Browne.
      [1913 Webster]

            One breast laid open were a school
            Which would unteach mankind the lust to shine or
            rule.                                 --Byron.
      [1913 Webster]

   2. To cause to be forgotten; as, to unteach what has been
      learned. --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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"