。
She's anything but polite.
Literal
She [topic-は] never courteous-not.
決して...ない is the strongly emphatic negation 'never / by no means / absolutely not.' The combination is a fixed pair — using 決して without a negative predicate would feel ungrammatical. 礼儀正しい ('polite, well-mannered, courteous') literally parses as '礼儀 (etiquette) + 正しい (correct)' — 'correctly observing etiquette,' a more weighty word than the simple 丁寧 ('polite' as a manner of speech). Behavior described as not 礼儀正しい reflects on the person's character broadly, not just their phrasing.