The Collaborative International Dictionary
prescriptivist
prescriptivist \pre*scrip"tiv*ist\, n. A person who believes that acceptable practices should be prescribed by an authority rather than be determined by the usage of the general public; especially, a supporter of prescriptive[2] rules of grammar; -- also used attributively, as prescriptivist grammar.
Wiktionary
prescriptivist
a. Having a tendency to prescribe. n. Someone who lays down rules regarding language usage, or who believes that traditional norms of language usage should be upheld.