Is there a word for a person who describes prescriptivists?

That depends on whether you describe them favourably or unfavourably. If unfavourably, the word is "astute".

MISTAKES, FALLACIES, AND IRRESPONSIBILITES OF PRESCRIPTIVE GRAMMAR

Extract:

Prescriptive grammarians are adamant, and their forceful prescriptions and high-brow judgments are irresponsible, and a denial of the rich cultural heritage of our language.

Descriptive grammarians, those who think and attempt to promote that usage rules should be based upon the more reasonable precedents set forth in the language's dialects and history (sometimes as far back as Old English), fight a seemingly endless battle against the established norms of prescriptivists. However, descriptivists can use a plethora of arguments to point out the many logical fallacies of a language with an improper usage base in Latin.

Hot linguistic debate often occurs over a number of normative usage rules. One example which leaps instantly to mind is the foolish "one must never split an infinitive." In Lowth's grammar infinitives cannot be split. It is not possible for Lowth because it is not possible in Latin to split an infinitive. Well, of course not. In Latin, an infinitive is one word. However, it is not in English. English infinitives are two words, such as "to split," and there is little logic to keeping them fused together, except that it cannot be done in Latin and Bishop Lowth decided, quite on his own, that English should emulate Latin, and the world followed suit. Thus, one foolish man has made a messy mockery of the rich and dynamic English language. Because of Lowth's erroneous decision, users of English have no end of confusion and difficulty sorting out these illogical rules.

http://www.newdream.net/~scully/toelw/Lowth.htm