The Collaborative International Dictionary of English v.0.48:

Thwart \Thwart\, v. t. [imp. & p. p. Thwarted; p. pr. & vb. n.
   Thwarting.]
   1. To move across or counter to; to cross; as, an arrow
      thwarts the air. [Obs.]
      [1913 Webster]

            Swift as a shooting star
            In autumn thwarts the night.          --Milton.
      [1913 Webster]

   2. To cross, as a purpose; to oppose; to run counter to; to
      contravene; hence, to frustrate or defeat.
      [1913 Webster]

            If crooked fortune had not thwarted me. --Shak.
      [1913 Webster]

            The proposals of the one never thwarted the
            inclinations of the other.            --South.
      [1913 Webster]

WordNet (r) 3.0 (2006):

thwarting
    adj 1: preventing realization or attainment of a desire [syn:
           frustrating, frustrative, thwarting]
    n 1: an act of hindering someone's plans or efforts [syn:
         frustration, thwarting, foiling]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "thwarting":
   annulment, bafflement, balk, balking, buck-passing, cancellation,
   check, checkmate, circumvention, confounding, counterbalancing,
   defeat, discomfiture, disconcertion, elusion, evasion, foil,
   foiling, frustration, getting around, getting round, invalidation,
   neutralization, nullification, offsetting, outguessing,
   outmaneuvering, outwitting, passing the buck, the runaround,
   the slip, undoing, upset, vitiation, voiding