The Collaborative International Dictionary of English v.0.48:

Undesirable \Undesirable\
   See desirable.

WordNet (r) 3.0 (2006):

undesirable
    adj 1: not wanted; "undesirable impurities in steel";
           "legislation excluding undesirable aliens";"removed the
           unwanted vegetation" [syn: undesirable, unwanted]
           [ant: desirable]
    2: not worthy of being chosen (especially as a spouse) [syn:
       undesirable, unsuitable]
    n 1: one whose presence is undesirable; "rounding up vagrants
         and drunks and other undesirables"

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "undesirable":
   DP, Ishmael, bad, bad child, bad example, bad man, bad news,
   bad person, bad woman, bitter, castaway, declasse, derelict,
   disagreeable, dislikable, disliked, displaced person, displeasing,
   disreputable, disreputable person, distasteful, evictee,
   exceptionable, exile, expatriate, expellee, futile, ill-advised,
   ill-considered, ill-favored, ill-suited, ill-timed, impolitic,
   impossible, improper, inadmissible, inadvisable, inappropriate,
   inapt, incongruous, indefensible, inept, inexpedient, infelicitous,
   inopportune, intolerable, leper, mal a propos, malapropos,
   objectionable, objectionable person, obnoxious, offensive,
   out of place, outcast, outcast of society, outcaste, outlaw,
   pariah, persona non grata, reject, repugnant, social outcast, sour,
   thankless, ugly, unacceptable, unacceptable person, unagreeable,
   unalluring, unappealing, unappetizing, unasked, unattractive,
   unbecoming, unbefitting, unbidden, undelectable, undelicious,
   unengaging, unenjoyable, unfit, unfitting, unfortunate, unhappy,
   uninvited, uninviting, unlikable, unmeet, unpalatable, unpleasant,
   unpleasing, unprofitable, unsavory, unseasonable, unseemly,
   unsuitable, untasteful, untenable, untimely, untouchable, unwanted,
   unwelcome, unwise, unworthy, wrong