WordNet (r) 3.0 (2006):

root out
    v 1: pull up by or as if by the roots; "uproot the vine that has
         spread all over the garden" [syn: uproot, extirpate,
         deracinate, root out]
    2: destroy completely, as if down to the roots; "the vestiges of
       political democracy were soon uprooted" "root out corruption"
       [syn: uproot, eradicate, extirpate, root out,
       exterminate]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "root out":
   abate, abolish, abscind, abstract, amputate, annihilate, avulse,
   ban, bar, blot out, bob, buck off, cast off, cast out, chuck,
   clear, clear away, clear out, clear the decks, clip, crop, cull,
   cut, cut away, cut off, cut out, demolish, deport, depose,
   deracinate, destroy, dig out, dig up, disentangle, dislodge,
   dismount, dispose of, dock, draw, draw out, dredge, dredge up,
   eject, elide, eliminate, enucleate, eradicate, evolve, evulse,
   excavate, except, excise, exclude, exile, expatriate, expel,
   exsect, exterminate, extinguish, extirpate, extract, extricate,
   ferret out, fish out, get out, get quit of, get rid of,
   get shut of, gouge out, grub up, hunt out, isolate, knock off,
   liquidate, lop, mine, mutilate, nip, outlaw, pare, peel, pick out,
   pluck out, pluck up, prune, pry out, pull, pull out, pull up,
   purge, quarry, rake out, raze, remove, rip out, root up, rule out,
   scout out, search out, set apart, set aside, shave, shear, spy out,
   stamp out, strike off, strike out, strip, strip off, sweep away,
   take off, take out, tear out, throw off, throw over,
   throw overboard, truncate, unearth, unhorse, unplace, unravel,
   unsaddle, unseat, uproot, weed out, wipe out, withdraw,
   wrest out