The Collaborative International Dictionary of English v.0.48:

Unwarrantable \Un*war"rant*a*ble\, a.
   Not warrantable; indefensible; not vindicable; not
   justifiable; illegal; unjust; improper. --
   Un*war"rant*a*ble*ness, n. -- Un*war"rant*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unwarrantable
    adj 1: incapable of being justified or explained [syn:
           indefensible, insupportable, unjustifiable,
           unwarrantable, unwarranted]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "unwarrantable":
   actionable, against the law, anarchic, anarchistic, anomic,
   black-market, bootleg, chargeable, contraband, contrary to law,
   criminal, felonious, flawed, illegal, illegitimate, illicit,
   impermissible, indefensible, inexcusable, inexpiable, irregular,
   irremissible, justiciable, lawless, nonconstitutional, nonlegal,
   nonlicit, outlaw, outlawed, punishable, triable, unallowable,
   unallowed, unauthorized, unconscionable, unconstitutional,
   under-the-counter, under-the-table, undue, unforgivable,
   unjustifiable, unlawful, unofficial, unpardonable, unreasonable,
   unstatutory, unwarranted, wrongful