Preventer.
To stop or hinder, as the approach of a person, or the performance of something; to impede; to obstruct; in old usage, to go before; to anticipate.
Preventable, preventible.
Who can prevent it? "The Pool in the Desert", Sara Jeanette Duncan
Who can prevent it?
Soldiers prevent battles, sometimes. "His Big Opportunity", Amy Le Feuvre
Soldiers prevent battles, sometimes.
What can you do to prevent it? "Down the Slope", James Otis
What can you do to prevent it?
An enormous bird in Persian folk- tales. ...