The Collaborative International Dictionary of English v.0.48:

decollate \de*col"late\ (d[-e]*k[o^]l"l[=a]t), v. t. [imp. & p.
   p. Decollated; p. pr. & vb. n. Decollating.] [L.
   decollatus, p. p. of decollare to behead; de- + collum neck.]
   To sever from the neck; to behead; to decapitate.
   [1913 Webster]

         The decollated head of St. John the Baptist. --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

decollate
    v 1: cut the head of; "the French King was beheaded during the
         Revolution" [syn: decapitate, behead, decollate]