The Collaborative International Dictionary of English v.0.48:

Extortioner \Ex*tor"tion*er\, n.
   One who practices extortion; an extortionist.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extortioner
    n 1: a criminal who extorts money from someone by threatening to
         expose embarrassing information about them [syn:
         blackmailer, extortioner, extortionist]