The Collaborative International Dictionary of English v.0.48:

Obstruct \Ob*struct"\, v. t. [imp. & p. p. Obstructed; p. pr.
   & vb. n. Obstructing.] [L. obstructus, p. p. of obstruere
   to build up before or against, to obstruct; ob (see Ob-) +
   struere to pile up. See Structure.]
   [1913 Webster]
   1. To block up; to stop up or close, as a way or passage; to
      place an obstacle in, or fill with obstacles or
      impediments that prevent or hinder passing; as, to
      obstruct a street; to obstruct the channels of the body.
      [1913 Webster]

            'T is the obstructed paths of sound shall clear.
                                                  --Pope.
      [1913 Webster]

   2. To be, or come, in the way of; to hinder from passing; to
      stop; to impede; to retard; as, the bar in the harbor
      obstructs the passage of ships; clouds obstruct the light
      of the sun; unwise rules obstruct legislation. "Th'
      impatience of obstructed love." --Johnson.
      [1913 Webster]

   Syn: To bar; barricade; stop; arrest; check; interrupt; clog;
        choke; impede; retard; embarrass; oppose.
        [1913 Webster]

WordNet (r) 3.0 (2006):

obstruct
    v 1: hinder or prevent the progress or accomplishment of; "His
         brother blocked him at every turn" [syn: obstruct,
         blockade, block, hinder, stymie, stymy,
         embarrass]
    2: block passage through; "obstruct the path" [syn: obstruct,
       obturate, impede, occlude, jam, block, close up]
       [ant: disengage, free]
    3: shut out from view or get in the way so as to hide from
       sight; "The thick curtain blocked the action on the stage";
       "The trees obstruct my view of the mountains" [syn:
       obstruct, block]

Moby Thesaurus II by Grady Ward, 1.0:

130 Moby Thesaurus words for "obstruct":
   arrest, backpedal, backwater, bar, barricade, beat off, bind,
   block, block out, block up, blockade, bolt, brake, bung, caulk,
   check, chink, chock, choke, choke off, choke up, clip the wings,
   clog, clog up, close, close off, close tight, close up, confine,
   congest, constipate, constrict, cork, counter, cover, crowd, curb,
   dam, dam up, debar, decelerate, delay, detain, dog, drag,
   draw rein, drive back, ease off, ease up, fend, fend off, fill,
   fill up, forbid, foul, halt, hamper, hinder, hold at bay,
   hold back, hold in check, hold off, hold up, impede,
   interfere with, interrupt, jam, keep at bay, keep back, keep off,
   lag, let down, let up, lock, lose ground, lose momentum,
   lose speed, make late, moderate, obstipate, occlude, pack, parry,
   plug, plug up, preclude, prevent, prohibit, push back, put back,
   rebuff, reef, rein in, relax, repel, repulse, retard, set back,
   shroud, shut off, shut out, shut tight, slack off, slack up,
   slacken, slow, slow down, slow up, spile, squeeze, squeeze shut,
   stall, stanch, stave off, stay, stench, stifle, stop, stop up,
   stopper, stopple, strangle, strangulate, stuff, stuff up,
   suffocate, take in sail, throttle down, turn aside, ward off