The Collaborative International Dictionary of English v.0.48:
Preventer \Pre*vent"er\, n.
1. One who goes before; one who forestalls or anticipates
another. [Obs.] --Bacon.
2. One who prevents or obstructs; a hinderer; that which
hinders; as, a preventer of evils or of disease.
3. (Naut.) An auxiliary rope to strengthen a mast.
Preventer bolts, or Preventer plates (Naut.), fixtures
connected with preventers to re["e]nforce other rigging.
Preventer stay. (Naut.) Same as Preventer, 3.