The Collaborative International Dictionary of English v.0.48:

Misuser \Mis*us"er\, n.
   1. One who misuses. "Wretched misusers of language."
      --Coleridge.
      [1913 Webster]

   2. (Law) Unlawful use of a right; use in excess of, or
      varying from, one's right. --Bouvier.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

MISUSER. An unlawful use of a right.
     2. In cases of public officers and corporations, a misuser is
sufficient to cause the right to be forfeited. 2 Bl. Com. 153; 5 Pick. R.
163.