The Collaborative International Dictionary of English v.0.48:

Befoul \Be*foul"\, v. t. [imp. & p. p. Befouled; p. pr. & vb.
   n. Befouling.] [Cf. AS. bef?lan; pref. be- + f?lan to foul.
   See Foul, a.]
   1. To make foul; to soil.
      [1913 Webster]

   2. To entangle or run against so as to impede motion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

befoul
    v 1: spot, stain, or pollute; "The townspeople defiled the river
         by emptying raw sewage into it" [syn: foul, befoul,
         defile, maculate]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "befoul":
   abuse, afflict, aggrieve, benasty, bespatter, bewitch, blacken,
   blight, condemn, contaminate, convert, corrupt, crucify, curse,
   damage, debase, defalcate, defame, defile, denigrate, deprave,
   desecrate, despoil, destroy, disadvantage, disserve, distress,
   divert, do a mischief, do evil, do ill, do wrong, do wrong by,
   doom, embezzle, envenom, foul, get into trouble, harass, harm, hex,
   hurt, impair, infect, injure, jinx, maladminister, maltreat,
   menace, mess, mess up, misapply, misappropriate, misemploy,
   mishandle, mismanage, mistreat, misuse, molest, nasty, outrage,
   peculate, persecute, pervert, pilfer, play havoc with,
   play hob with, poison, pollute, prejudice, profane, prostitute,
   savage, scathe, slander, slur, smear, spatter, sully, taint,
   tarnish, threaten, torment, torture, traduce, violate, wound,
   wreak havoc on, wrong