Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
prō-scriptor, ōris, m. [proscribo], one who proscribes or oullaws; adj., proscriptive (post-Aug.): proscriptor animus, Plin. 7, 12, 10, § 56; cf. proscripturio.