The Collaborative International Dictionary of English v.0.48:

Unsuitable \Unsuitable\
   See suitable.

WordNet (r) 3.0 (2006):

unsuitable
    adj 1: not meant or adapted for a particular purpose; "a solvent
           unsuitable for use on wood surfaces"
    2: not capable of being applied; "rules inapplicable to day
       students" [syn: inapplicable, unsuitable]
    3: not conducive to good moral development; "the movie is
       unsuitable for children"
    4: not worthy of being chosen (especially as a spouse) [syn:
       undesirable, unsuitable]

Moby Thesaurus II by Grady Ward, 1.0:

144 Moby Thesaurus words for "unsuitable":
   aberrant, abnormal, abominable, atrocious, bad, chintzy, criminal,
   delinquent, deviant, disgraceful, evil, exceptionable,
   functionless, futile, hardly the thing, ignominious, ill-adapted,
   ill-advised, ill-assorted, ill-chosen, ill-considered, ill-fitted,
   ill-matched, ill-seasoned, ill-sorted, ill-suited, ill-timed,
   illegal, impolitic, impossible, improper, in bad taste,
   in disrepair, inadmissible, inadvisable, inapplicable, inapposite,
   inappropriate, inapt, inauspicious, incongruous, inconvenient,
   incorrect, indecent, indecorous, indefensible, indelicate,
   inelegant, inept, inexpedient, infamous, infelicitous, inoperable,
   inoperative, inopportune, intempestive, intolerable, intrusive,
   irrelevant, late, mal a propos, maladjusted, malapropos, misjoined,
   mismatched, mismated, misplaced, mistimed, nonfunctional,
   nonutilitarian, not done, not the thing, objectionable, off base,
   off-base, off-color, offensive, otiose, out of character,
   out of joint, out of keeping, out of line, out of order,
   out of phase, out of place, out of proportion, out of season,
   out of time, out of tune, out of whack, out-of-line, premature,
   sacrilegious, scandalous, shameful, shameless, sinful, tasteless,
   terrible, too late, too soon, unacceptable, unadapted, unapt,
   unbecoming, unbefitting, unbeseeming, unconducive, undesirable,
   undignified, undue, unemployable, unfavorable, unfit, unfitted,
   unfitting, unfortunate, ungenteel, unhandy, unhappy, unhelpful,
   unlawful, unlucky, unmeet, unprofitable, unpropitious, unqualified,
   unready, unrighteous, unripe, unseasonable, unseemly, unsuited,
   untasteful, untenable, untimely, untoward, unusable, unwise,
   unworkable, vulgar, wicked, wrong, wrongful