The Collaborative International Dictionary of English v.0.48:

Block \Block\ (bl[o^]k), v. t. [imp. & p. p. Blocked
   (bl[o^]kt); p. pr. & vb. n. Blocking.] [Cf. F. bloquer, fr.
   bloc block. See Block, n.]
   1. To obstruct so as to prevent passage or progress; to
      prevent passage from, through, or into, by obstructing the
      way; -- used both of persons and things; -- often followed
      by up; as, to block up a road or harbor; to block an
      entrance.
      [1913 Webster]

            With moles . . . would block the port. --Rowe.
      [1913 Webster]

            A city . . . besieged and blocked about. --Milton.
      [1913 Webster]

   2. To secure or support by means of blocks; to secure, as two
      boards at their angles of intersection, by pieces of wood
      glued to each.
      [1913 Webster]

   3. To shape on, or stamp with, a block; as, to block a hat.
      [1913 Webster]

   4. to cause (any activity) to halt by creating an
      obstruction; as, to block a nerve impulse; to block a
      biochemical reaction with a drug.
      [PJC]

   To block out, to begin to reduce to shape; to mark out
      roughly; to lay out; to outline; as, to block out a plan.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

blocked \blocked\ adj.
   1. closed to traffic. Traffic was blocked by an overturned
      tractor-trailor

   Syn: out of use(predicate).
        [WordNet 1.5]

   2. at a complete standstill because of opposition of two
      unrelenting forces or factions.

   Syn: deadlocked, stalemated.
        [WordNet 1.5]

   3. unusable because of some obstruction.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

blocked
    adj 1: closed to traffic; "the repaving results in many blocked
           streets" [syn: blocked, out of use(p)]
    2: completely obstructed or closed off; "the storm was
       responsible for many blocked roads and bridges"; "the drain
       was plugged" [syn: blocked, plugged]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "blocked":
   Lethean, absentminded, amnestic, arrested, back, backward,
   behindhand, belated, bound, choked, choked up, clogged, clogged up,
   congested, constipated, converted, costive, delayed,
   delayed-action, detained, forgetful, forgetting, foul, fouled,
   full, heedless, held up, hung up, in a bind, in abeyance,
   inclined to forget, infarcted, jammed, late, latish, moratory,
   never on time, oblivious, obstipated, obstructed, overdue, packed,
   plugged, plugged up, repressed, retarded, slow, stopped,
   stopped up, stuffed, stuffed up, suppressed, tardy, unmindful,
   unpunctual, unready, untimely