noun. 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.

noun. Someone who lays down rules regarding language usage, or who believes that traditional norms of language usage should be upheld.

adjective. Having a tendency to prescribe.

"Likewise, to the dismay of us linguists, National Grammar Day will mostly just result in prescriptivist dilettantes coming out in full force, tossing around ignorant grammatical proclamations with gusto, like so many dimes at a dime toss."

descriptivist  

prescribe  prescriptivism