The Collaborative International Dictionary of English v.0.48:

Intractable \In*tract"a*ble\, a. [L. intractabilis: cf. F.
   intraitable, formerly also intractable. See In- not, and
   Tractable.]
   Not tractable; not easily governed, managed, or directed;
   indisposed to be taught, disciplined, or tamed; violent;
   stubborn; obstinate; refractory; as, an intractable child.

   Syn: Stubborn; perverse; obstinate; refractory; cross;
        unmanageable; unruly; headstrong; violent; ungovernable;
        unteachable. -- In*tract"a*ble*ness, n. --
        In*tract"a*bly, adv.
        [1913 Webster]

WordNet (r) 3.0 (2006):

intractable
    adj 1: not tractable; difficult to manage or mold; "an
           intractable disposition"; "intractable pain"; "the most
           intractable issue of our era"; "intractable metal" [ant:
           manipulable, tractable]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "intractable":
   adamant, adamantine, beyond control, breachy, bullheaded,
   contumacious, defiant, fractious, headstrong, immovable, immutable,
   incorrigible, indocile, indomitable, inductile, inelastic,
   inextensible, inextensile, inextensional, inflexible,
   insubordinate, insuppressible, intractile, intransigent,
   irrepressible, irresilient, mulish, noncooperating, noncooperative,
   nonelastic, nonstretchable, obstreperous, out of hand,
   pertinacious, perverse, recalcitrant, refractory, resistant,
   resisting, restive, self-willed, shrewish, stubborn, tough,
   unalterable, unbending, unbiddable, uncompliant, uncontrollable,
   uncooperative, undisciplined, unextendible, unextensible,
   unflexible, ungiving, ungovernable, unlimber, unmalleable,
   unmanageable, unmoldable, unpliable, unpliant, unruly,
   unsubmissive, untractable, unyielding, wild, willful