The Collaborative International Dictionary of English v.0.48:

Immobilize \Im*mob"i*lize\, v. t. [Pref. im- in + mobilize; cf.
   f. immobiliser.]
   To make immovable; in surgery, to make immovable (a naturally
   mobile part, as a joint) by the use of splints, or stiffened
   bandages.
   [1913 Webster]

WordNet (r) 3.0 (2006):

immobilize
    v 1: hold as reserve or withdraw from circulation; of capital
         [syn: immobilize, immobilise]
    2: to hold fast or prevent from moving; "The child was pinned
       under the fallen tree" [syn: trap, pin, immobilize,
       immobilise]
    3: make defenseless [syn: immobilize, immobilise]
    4: convert (assets) into fixed capital [syn: immobilize,
       immobilise]
    5: prohibit the conversion or use of (assets); "Blocked funds";
       "Freeze the assets of this hostile government" [syn:
       freeze, block, immobilize, immobilise] [ant: free,
       release, unblock, unfreeze]
    6: cause to be unable to move; "The sudden storm immobilized the
       traffic" [syn: immobilize, immobilise]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "immobilize":
   balance, ballast, counterbalance, cripple, disable, disarm, firm,
   firm up, freeze, hold, incapacitate, keep, nail down, pin down,
   prostrate, retain, stabilitate, stabilize, steady, stick,
   transfix