The Collaborative International Dictionary of English v.0.48:

Unmovable \Un*mov"a*ble\, a.
   Immovable. "Steadfast, unmovable." --1 Cor. xv. 58. Locke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unmovable
    adj 1: not able or intended to be moved; "the immovable hills"
           [syn: immovable, immoveable, stabile, unmovable]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "unmovable":
   adamant, adamantine, at a standstill, closed to, firm, frozen,
   immobile, immotile, immotive, immovable, impersuadable,
   impersuasible, impervious, inflexible, irremovable, obstinate, pat,
   rigid, standpat, stationary, unamenable, unimpressionable,
   uninfluenceable, unmoving, unpersuadable, unpliable, unreceptive,
   unresponsive, unsuggestible, unsusceptible, unswayable,
   unyielding