The Collaborative International Dictionary of English v.0.48:

dethrone \de*throne"\ (d[-e]*thr[=o]n"), v. t. [imp. & p. p.
   Dethroned (d[-e]*thr[=o]nd"); p. pr. & vb. n.
   Dethroning.] [Pref. de- + throne: cf. F. d['e]tr[^o]ner;
   pref. d['e]- (L. dis-) + tr[^o]ne throne. See Throne.]
   To remove or drive from a throne; to depose; to divest of
   supreme authority and dignity. "The Protector was dethroned."
   --Hume.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dethrone
    v 1: remove a monarch from the throne; "If the King does not
         abdicate, he will have to be dethroned" [ant: enthrone,
         throne]