The Collaborative International Dictionary of English v.0.48:

Unteachable \Unteachable\
   See teachable.

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "unteachable":
   Boeotian, asinine, beef-brained, beef-witted, blockish, bovine,
   chumpish, cloddish, cowish, crass, dense, doltish, dullard, dumb,
   duncical, duncish, fat, gross, ineducable, klutzy, lumpish, oafish,
   opaque, sottish, stupid, thick, wrongheaded